cmd/go: don't let ... match GOROOT/src/cmd in module mode

GOROOT/src/cmd uses GOROOT/src/cmd/vendor, which module
mode simply cannot handle.

Exposed by making ... match the standard library, which it still should.

But for now it's fine to just exclude commands.

Change-Id: I2201b94445f11239022de8a2473aa3b573f405c0
Reviewed-on: https://go-review.googlesource.com/129055
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/modload/search.go

@@ -37,6 +37,10 @@ func matchPackages(pattern string, tags map[string]bool, useStd bool, modules []
 
 	walkPkgs := func(root, importPathRoot string) {
 		root = filepath.Clean(root)
+		var cmd string
+		if root == cfg.GOROOTsrc {
+			cmd = filepath.Join(root, "cmd")
+		}
 		filepath.Walk(root, func(path string, fi os.FileInfo, err error) error {
 			if err != nil {
 				return nil
@@ -47,6 +51,14 @@ func matchPackages(pattern string, tags map[string]bool, useStd bool, modules []
 				return nil
 			}
 
+			// GOROOT/src/cmd makes use of GOROOT/src/cmd/vendor,
+			// which module mode can't deal with. Eventually we'll stop using
+			// that vendor directory, and then we can remove this exclusion.
+			// golang.org/issue/26924.
+			if path == cmd {
+				return filepath.SkipDir
+			}
+
 			want := true
 			// Avoid .foo, _foo, and testdata directory trees.
 			_, elem := filepath.Split(path)

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

@@ -15,6 +15,7 @@ stdout '^unsafe$'
 ! stdout index/suffixarray
 
 # 'go list ...' should list packages in all active modules and the standard library.
+# But not cmd/* - see golang.org/issue/26924.
 go list ...
 stdout example.com/unused/useerrors
 stdout example.com/m/useunsafe
@@ -23,6 +24,7 @@ stdout example.com/m/useunsafe
 stdout '^unicode$'
 stdout '^unsafe$'
 stdout index/suffixarray
+! stdout cmd/pprof
 
 # 'go list example.com/m/...' should list packages in all modules that begin with
 # "example.com/m/".