cmd/go: use package index for std in load.loadPackageData

cmd/go: use package index for std in load.loadPackageData

load.loadPackageData was only using an index for modules,
not for standard library packages. Other parts of the code were
using the index, so there was some benefit, but not as much
as you'd hope.

With the index disabled, the Script/work test takes 2.2s on my Mac.
With the index enabled before this CL, it took 2.0s.
With the index enabled after this CL, it takes 1.6s.

Before this CL, the Script/work test issued:

	 429 IsDir
	  19 IsDirWithGoFiles
	   7 Lstat
	9072 Open
	 993 ReadDir
	 256 Stat
	   7 Walk
	   3 indexModule
	  24 openIndexModule
	 525 openIndexPackage

After this CL, it issued:

	  19 IsDirWithGoFiles
	   7 Lstat
	  60 Open
	 606 ReadDir
	 256 Stat
	   7 Walk
	   3 indexModule
	  24 openIndexModule
	 525 openIndexPackage

This speedup helps the Dragonfly builder, which has very slow
file I/O and is timing out since a recent indexing change.

Times for go test -run=Script/^work$ on the Dragonfly builder:

	50s before indexing changes
	31s full module indexing of std
	46s per-package indexing of std

It cuts the time for go test -run=Script/^work$ from 44s to 20s.

For #53577.

Change-Id: I7189a77fc7fdf61de3ab3447efc4e84d1fc52c25
Reviewed-on: https://go-review.googlesource.com/c/go/+/416134
Reviewed-by: Bryan Mills <bcmills@google.com>