cmd/go/internal/modload: fail if no package was found in local module

Changing the Import function to return a PackageNotInModuleError if no
package was found in a local module. This replacing the vague message
"missing dot in first path element" you get today with much more friendly
one - "module was found, but does not contain package".

Fixes #35273

Change-Id: I6d726c17e6412258274b10f58f76621617d26e0a
Reviewed-on: https://go-review.googlesource.com/c/go/+/203118
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modload/import.go

@@ -237,6 +237,17 @@ func Import(path string) (m module.Version, dir string, err error) {
 				return m, "", &ImportMissingError{Path: path, Module: m}
 			}
 		}
+		if len(mods) > 0 && module.CheckPath(path) != nil {
+			// The package path is not valid to fetch remotely,
+			// so it can only exist if in a replaced module,
+			// and we know from the above loop that it is not.
+			return module.Version{}, "", &PackageNotInModuleError{
+				Mod:         mods[0],
+				Query:       "latest",
+				Pattern:     path,
+				Replacement: Replacement(mods[0]),
+			}
+		}
 	}
 
 	candidates, err := QueryPackage(path, "latest", Allowed)

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

@@ -23,6 +23,13 @@ stdout 'example.com/y v0.0.0-00010101000000-000000000000 => ./y'
 stdout 'example.com/x/v3 v3.0.0-00010101000000-000000000000 => ./v3'
 stdout 'example.com/v v1.12.0 => ./v12'
 
+# The go command should print an informative error when the matched
+# module does not contain a package.
+cd fail
+! go list all
+stdout 'localhost.fail'
+stderr '^can.t load package: m.go:3:8: module w@latest found \(v0.0.0-00010101000000-000000000000, replaced by ../w\), but does not contain package w$'
+
 -- go.mod --
 module example.com/m
 
@@ -107,3 +114,16 @@ package v
 module v.localhost
 -- v/v.go --
 package v
+
+-- fail/m.go --
+package main
+
+import _ "w"
+
+func main() {}
+
+-- fail/go.mod --
+module localhost.fail
+
+replace w => ../w
+