path/filepath: Clean(`.\c:`) returns `c:` on Windows [1.17 backport]

[gopherbot]

@ianlancetaylor requested issue #52476 to be considered for backport to the next 1.17 minor release.

@gopherbot Please open backport issues.

Avoid confusion with ./C: paths.

[gopherbot]

Change https://go.dev/cl/405235 mentions this issue: [release-branch.go1.17] path/filepath: do not remove prefix "." when following path contains ":"

[heschi]

cc @golang/security to review the fix

[gopherbot]

Closed by merging 4c69fd5 to release-branch.go1.17.