cmd/go: go mod vendor: open C:\Users\LICENSE: Access is denied. [1.15 backport]

[gopherbot]

@bcmills requested issue #46867 to be considered for backport to the next 1.15 minor release.

@gopherbot, please backport. This bug has a trivial fix, and the failure mode is fairly confusing with no obvious workaround.

[gopherbot]

Change https://golang.org/cl/332330 mentions this issue: [release-branch.go1.15] cmd/go: use path.Dir instead of filepath.Dir for package paths in 'go mod vendor'

[gopherbot]

Closed by merging 1b68e07 to release-branch.go1.15.