cmd/go/internal/load: use file path prefix check for GOROOT dirs

When loading package data in module mode, loadPackageData tries to
detect whether a resolved directory is under GOROOT/src so it can use
the package index for standard-library packages. That check used
str.HasPathPrefix, which is for slash-separated import paths. On
Windows, r.dir and cfg.GOROOTsrc are filesystem paths using backslashes,
so the check failed and standard-library package loads fell back to
go/build.ImportDir.

Use str.HasFilePathPrefix for those filesystem path checks. This lets
Windows use the package index for GOROOT package loads instead of
reopening standard library source files.

Measured on windows/arm64 with:

    go list -e -compiled=true -test=true -export=false -deps=true -find=false -pgo=off ./...

after the separate go list -compiled optimization, this reduced runtime
from about 690 ms to about 424 ms, a 39% improvement.

A gofsystrace run on the same command showed source-file opens dropping
from 1258 to 5.

For #75238.

Change-Id: I34a9faa846516726f80f5d12bb8ac374e0fe99a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/787340
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Ian Alexander <jitsu@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@google.com>