os: use filepathlite.Base
Replace custom basename implementations with filepathlite.Base across
all relevant os/stat files to unify path processing across platforms.
Change-Id: I7c4795661926949bae71e66d8b4f9363e7caef15
GitHub-Last-Rev:
1236e93ebcd4137f9cbbbab2163cadf4e4d02674
GitHub-Pull-Request: golang/go#67195
Reviewed-on: https://go-review.googlesource.com/c/go/+/583415
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>