cmd/go: disambiguate for-test packages in failure output

Now that we can tell when a package is a split-off copy
for testing, show that in the build failures.
For example, instead of:

	# regexp/syntax
	../../regexp/syntax/parse.go:9:2: can't find import: "strings"
	# path/filepath
	../../path/filepath/match.go:12:2: can't find import: "strings"
	# flag
	../../flag/flag.go:75:2: can't find import: "strings"

we now print

	# regexp/syntax [strings.test]
	../../regexp/syntax/parse.go:9:2: can't find import: "strings"
	# path/filepath [strings.test]
	../../path/filepath/match.go:12:2: can't find import: "strings"
	# flag [strings.test]
	../../flag/flag.go:75:2: can't find import: "strings"

which gives more of a hint about what is wrong.

This is especially helpful if a package is being built multiple times,
since it explains why an error might appear multiple times:

	$ go test regexp encoding/json
	# regexp
	../../regexp/exec.go:12:9: undefined: x
	# regexp [regexp.test]
	../../regexp/exec.go:12:9: undefined: x
	FAIL	regexp [build failed]
	FAIL	encoding/json [build failed]
	$

Change-Id: Ie325796f6c3cf0e23f306066be8e65a30cb6b939
Reviewed-on: https://go-review.googlesource.com/108155
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/load/pkg.go

@@ -122,6 +122,14 @@ func (p *Package) AllFiles() []string {
 	)
 }
 
+// Desc returns the package "description", for use in b.showOutput.
+func (p *Package) Desc() string {
+	if p.ForTest != "" {
+		return p.ImportPath + " [" + p.ForTest + ".test]"
+	}
+	return p.ImportPath
+}
+
 type PackageInternal struct {
 	// Unexported fields are not part of the public API.
 	Build        *build.Package

src/cmd/go/internal/work/exec.go

@@ -573,7 +573,7 @@ func (b *Builder) build(a *Action) (err error) {
 	objpkg := objdir + "_pkg_.a"
 	ofile, out, err := BuildToolchain.gc(b, a, objpkg, icfg.Bytes(), len(sfiles) > 0, gofiles)
 	if len(out) > 0 {
-		b.showOutput(a, a.Package.Dir, a.Package.ImportPath, b.processOutput(out))
+		b.showOutput(a, a.Package.Dir, a.Package.Desc(), b.processOutput(out))
 		if err != nil {
 			return errPrintedOutput
 		}
@@ -2411,13 +2411,13 @@ func (b *Builder) swigOne(a *Action, p *load.Package, file, objdir string, pcCFL
 			if bytes.Contains(out, []byte("-intgosize")) || bytes.Contains(out, []byte("-cgo")) {
 				return "", "", errors.New("must have SWIG version >= 3.0.6")
 			}
-			b.showOutput(a, p.Dir, p.ImportPath, b.processOutput(out)) // swig error
+			b.showOutput(a, p.Dir, p.Desc(), b.processOutput(out)) // swig error
 			return "", "", errPrintedOutput
 		}
 		return "", "", err
 	}
 	if len(out) > 0 {
-		b.showOutput(a, p.Dir, p.ImportPath, b.processOutput(out)) // swig warning
+		b.showOutput(a, p.Dir, p.Desc(), b.processOutput(out)) // swig warning
 	}
 
 	// If the input was x.swig, the output is x.go in the objdir.

src/cmd/go/internal/work/gc.go

@@ -295,7 +295,7 @@ func (gcToolchain) pack(b *Builder, a *Action, afile string, ofiles []string) er
 		return nil
 	}
 	if err := packInternal(absAfile, absOfiles); err != nil {
-		b.showOutput(a, p.Dir, p.ImportPath, err.Error()+"\n")
+		b.showOutput(a, p.Dir, p.Desc(), err.Error()+"\n")
 		return errPrintedOutput
 	}
 	return nil