internal/lsp/cache: fix loading of std and cmd

The go/packages queries std/... and cmd/... don't work. I believe
they're supposed to be just "std" and "cmd" based on experimentation
(and stdlib_test.go), but I didn't see this explicitly documented.

Fixes golang/go#46901

Change-Id: I3d0ed48fa64a50eefe4b5cc6074a93455cd37dc8
Reviewed-on: https://go-review.googlesource.com/c/tools/+/330529
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

internal/lsp/cache/load.go

@@ -87,7 +87,12 @@ func (s *snapshot) load(ctx context.Context, allowNetwork bool, scopes ...interf
 			}
 			query = append(query, fmt.Sprintf("file=%s", uri.Filename()))
 		case moduleLoadScope:
-			query = append(query, fmt.Sprintf("%s/...", scope))
+			switch scope {
+			case "std", "cmd":
+				query = append(query, string(scope))
+			default:
+				query = append(query, fmt.Sprintf("%s/...", scope))
+			}
 		case viewLoadScope:
 			// If we are outside of GOPATH, a module, or some other known
 			// build system, don't load subdirectories.