cmd/go/internal/lockedfile/internal/filelock: re-use filelock_other.go for plan9

cmd/go/internal/lockedfile/internal/filelock: re-use filelock_other.go for plan9

The implementation for lockType, lock, unlock and isNotSupported is the
same on plan9 as on other platforms where filelocks are unsupported.

Change-Id: I8b9c0bdc429e23346ab9145ec3814622319427fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/476915
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>