According to https://go.dev/issue/50478#issuecomment-1006947143, Plan
9 does not support renaming across directories. For the MultiModule
test cases we want to rename go.mod to testdata/go.mod, so we need to
fall back to copying if os.Rename fails.
Fixesgolang/go#50478.
Change-Id: I30b7601e1d4b122ce114015f3585f6ba6d6c9714
Reviewed-on: https://go-review.googlesource.com/c/tools/+/376115
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>