cmd/go: do not miss an error if import path contains "cmd/something"

Fixes #7638

LGTM=rsc
R=rsc, adg, robert.hencke, bradfitz
CC=golang-codereviews
https://golang.org/cl/89280043

src/cmd/go/get.go

@@ -140,6 +140,10 @@ var downloadRootCache = map[string]bool{}
 // for the package named by the argument.
 func download(arg string, stk *importStack, getTestDeps bool) {
 	p := loadPackage(arg, stk)
+	if p.Error != nil && p.Error.hard {
+		errorf("%s", p.Error)
+		return
+	}
 
 	// There's nothing to do if this is a package in the standard library.
 	if p.Standard {

src/cmd/go/pkg.go

@@ -139,6 +139,7 @@ type PackageError struct {
 	Pos           string   // position of error
 	Err           string   // the error itself
 	isImportCycle bool     // the error is an import cycle
+	hard          bool     // whether the error is soft or hard; soft errors are ignored in some places
 }
 
 func (p *PackageError) Error() string {
@@ -715,6 +716,7 @@ func loadPackage(arg string, stk *importStack) *Package {
 				Error: &PackageError{
 					ImportStack: stk.copy(),
 					Err:         fmt.Sprintf("invalid import path: cmd/... is reserved for Go commands"),
+					hard:        true,
 				},
 			}
 			return p