cmd/go: fix relative-path normalization in go.work files

We had been statting 'dir' instead of 'absDir', which statted the
wrong directory if 'dir' was made relative to the location of the
go.work file and that location was different from the current working
directory.

While we're here, I also noticed some other dir-vs.-absDir
discrepancies. The haveDirs map had assumed relative, slash-separated
filesystem paths, but absolute paths on Windows are not
slash-separated and we do retain those. Moreover, if users hand-edit
the go.work file they may introduce non-canonical paths.
I have changed the haveDirs logic to retain absolute paths in their
platform-specific forms, and to call DropUse with the original path
from the go.work file instead of the newly-resolved path.

Fixes #50931
Updates #48257

Change-Id: Ib0a46179aa20c99f045aac5c7c02dbb55da455c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/382240


Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>