cmd/go: make 'go get <module>@none' idempotent

Before this change, 'go get <module>@none' for a module not in the build list
would add the module to go.mod (with the explicit version string "none").
Subsequent go commands would fail with 'invalid module version "none"'.

Change-Id: Iebcaeab89eb19959f0a9aeda836f179962953313
Reviewed-on: https://go-review.googlesource.com/127215
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/modget/get.go

@@ -374,7 +374,7 @@ func runGet(cmd *base.Command, args []string) {
 	// Now we know the specific version of each path@vers.
 	// The final build list will be the union of three build lists:
 	//	1. the original build list
-	//	2. the modules named on the command line
+	//	2. the modules named on the command line (other than @none)
 	//	3. the upgraded requirements of those modules (if upgrading)
 	// Start building those lists.
 	// This loop collects (2).
@@ -395,8 +395,10 @@ func runGet(cmd *base.Command, args []string) {
 			continue // already added
 		}
 		byPath[t.m.Path] = t
+		if t.m.Version != "none" {
 			named = append(named, t.m)
 		}
+	}
 	base.ExitIfErrors()
 
 	// If the modules named on the command line have any dependencies

src/cmd/go/testdata/script/mod_get_none.txt

@@ -0,0 +1,12 @@
+env GO111MODULE=on
+
+go mod init example.com/foo
+
+# 'go get bar@none' should be a no-op if module bar is not active.
+go get example.com/bar@none
+go list -m all
+! stdout example.com/bar
+
+go get example.com/bar@none
+go list -m all
+! stdout example.com/bar