cmd/go: enable vendoring experiment by default

If we're going to do this for Go 1.6 we might as well do it now
and find out what breaks.

Change-Id: I8306b7829d8d13b564a1466c902ec6ba1a5a58c1
Reviewed-on: https://go-review.googlesource.com/13967
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/alldocs.go

@@ -484,8 +484,8 @@ rule is that if the local installation is running version "go1", get
 searches for a branch or tag named "go1". If no such version exists it
 retrieves the most recent version of the package.
 
-If the vendoring experiment is enabled (see 'go help gopath'),
-then when go get checks out or updates a Git repository,
+Unless vendoring support is disabled (see 'go help gopath'),
+when go get checks out or updates a Git repository,
 it also updates any git submodules referenced by the repository.
 
 For more about specifying packages, see 'go help packages'.
@@ -934,13 +934,10 @@ See https://golang.org/s/go14internal for details.
 
 Vendor Directories
 
-Go 1.5 includes experimental support for using local copies
-of external dependencies to satisfy imports of those dependencies,
-often referred to as vendoring. Setting the environment variable
-GO15VENDOREXPERIMENT=1 enables that experimental support.
+Go 1.6 includes support for using local copies of external dependencies
+to satisfy imports of those dependencies, often referred to as vendoring.
 
-When the vendor experiment is enabled,
-code below a directory named "vendor" is importable only
+Code below a directory named "vendor" is importable only
 by code in the directory tree rooted at the parent of "vendor",
 and only using an import path that omits the prefix up to and
 including the vendor element.
@@ -978,9 +975,18 @@ top-level "crash/bang".
 Code in vendor directories is not subject to import path
 checking (see 'go help importpath').
 
-When the vendor experiment is enabled, 'go get' checks out
-submodules when checking out or updating a git repository
-(see 'go help get').
+When 'go get' checks out or updates a git repository, it now also
+updates submodules.
+
+Vendor directories do not affect the placement of new repositories
+being checked out for the first time by 'go get': those are always
+placed in the main GOPATH, never in a vendor subtree.
+
+In Go 1.5, as an experiment, setting the environment variable
+GO15VENDOREXPERIMENT=1 enabled these features.
+As of Go 1.6 they are on by default. To turn them off, set
+GO15VENDOREXPERIMENT=0. In Go 1.7, the environment
+variable will stop having any effect.
 
 The vendoring semantics are an experiment, and they may change
 in future releases. Once settled, they will be on by default.

src/cmd/go/get.go

@@ -51,8 +51,8 @@ rule is that if the local installation is running version "go1", get
 searches for a branch or tag named "go1". If no such version exists it
 retrieves the most recent version of the package.
 
-If the vendoring experiment is enabled (see 'go help gopath'),
-then when go get checks out or updates a Git repository,
+Unless vendoring support is disabled (see 'go help gopath'),
+when go get checks out or updates a Git repository,
 it also updates any git submodules referenced by the repository.
 
 For more about specifying packages, see 'go help packages'.

src/cmd/go/help.go

@@ -365,13 +365,10 @@ See https://golang.org/s/go14internal for details.
 
 Vendor Directories
 
-Go 1.5 includes experimental support for using local copies
-of external dependencies to satisfy imports of those dependencies,
-often referred to as vendoring. Setting the environment variable
-GO15VENDOREXPERIMENT=1 enables that experimental support.
+Go 1.6 includes support for using local copies of external dependencies
+to satisfy imports of those dependencies, often referred to as vendoring.
 
-When the vendor experiment is enabled,
-code below a directory named "vendor" is importable only
+Code below a directory named "vendor" is importable only
 by code in the directory tree rooted at the parent of "vendor",
 and only using an import path that omits the prefix up to and
 including the vendor element.
@@ -409,9 +406,18 @@ top-level "crash/bang".
 Code in vendor directories is not subject to import path
 checking (see 'go help importpath').
 
-When the vendor experiment is enabled, 'go get' checks out
-submodules when checking out or updating a git repository
-(see 'go help get').
+When 'go get' checks out or updates a git repository, it now also
+updates submodules.
+
+Vendor directories do not affect the placement of new repositories
+being checked out for the first time by 'go get': those are always
+placed in the main GOPATH, never in a vendor subtree.
+
+In Go 1.5, as an experiment, setting the environment variable
+GO15VENDOREXPERIMENT=1 enabled these features.
+As of Go 1.6 they are on by default. To turn them off, set
+GO15VENDOREXPERIMENT=0. In Go 1.7, the environment
+variable will stop having any effect.
 
 The vendoring semantics are an experiment, and they may change
 in future releases. Once settled, they will be on by default.

src/cmd/go/pkg.go

@@ -250,11 +250,13 @@ func reloadPackage(arg string, stk *importStack) *Package {
 	return loadPackage(arg, stk)
 }
 
-// The Go 1.5 vendoring experiment is enabled by setting GO15VENDOREXPERIMENT=1.
+// The Go 1.5 vendoring experiment was enabled by setting GO15VENDOREXPERIMENT=1.
+// In Go 1.6 this is on by default and is disabled by setting GO15VENDOREXPERIMENT=0.
+// In Go 1.7 the variable will stop having any effect.
 // The variable is obnoxiously long so that years from now when people find it in
 // their profiles and wonder what it does, there is some chance that a web search
 // might answer the question.
-var go15VendorExperiment = os.Getenv("GO15VENDOREXPERIMENT") == "1"
+var go15VendorExperiment = os.Getenv("GO15VENDOREXPERIMENT") != "0"
 
 // dirToImportPath returns the pseudo-import path we use for a package
 // outside the Go path.  It begins with _/ and then contains the full path