cmd/go/internal/fsys: minor cleanup

cmd/go/internal/fsys: minor cleanup

Rename canonicalize to abs.
Rename IsDirWithGoFiles to IsGoDir.
Remove Init argument.
Split OverlayPath into Actual and Renamed.
Clean up doc comments.
Other minor cleanups.

Preparation for larger changes.

Change-Id: Ida022588149a1618a63acc91e3800b09df873b6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/628697
TryBot-Bypass: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>