cmd/go: do not index std as a module in modcache

We do not index std as a whole module ever.

When working in the main Go repo, files in package change often,
so we don't want to pay the cost of reindexing all of std when what
we really need is just to reindex strings. Per-package indexing
works better for that case.

When using a released Go toolchain, we don't have to worry about
the whole module changing, but if we switch to whole-module indexing
at that point, we have the potential for bugs that only happen in
released toolchains. Probably not worth the risk.

For similar reasons, we don't index the current work module as
a whole module (individual packages are changing), so we use the heuristic
that we only do whole-module indexing in the module cache.

The new toolchain modules live in the module cache, though, and
our heuristic was causing whole-module indexing for them.
As predicted, enabling whole-module indexing for std when it's
completely untested does in fact lead to bugs (a very minor one).

This CL turns off whole-module indexing for std even when it is
in the module cache, to bring toolchain module behavior back in
line with the other ways to run toolchains.

For #57001.

Change-Id: I5012dc713f566846eb4b2848facc7f75bc956eb9
Reviewed-on: https://go-review.googlesource.com/c/go/+/504119
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

Author: rsc

src/cmd/go/internal/modindex/read.go

@@ -160,7 +160,7 @@ func GetModule(modroot string) (*Module, error) {
 		return nil, errNotFromModuleCache
 	}
 	modroot = filepath.Clean(modroot)
-	if !str.HasFilePathPrefix(modroot, cfg.GOMODCACHE) {
+	if str.HasFilePathPrefix(modroot, cfg.GOROOTsrc) || !str.HasFilePathPrefix(modroot, cfg.GOMODCACHE) {
 		return nil, errNotFromModuleCache
 	}
 	return openIndexModule(modroot, true)