imports: make filepath.Rel work on windows

filepath.Rel will always fail if one parameter starts with
drive letter and the other does not. Make both filepath.Rel
parameters absolute paths, to give it chance to succeed.

Fixes broken tools build on windows.

Change-Id: Ibefcfe283f28977503323f01bc3a698478227f84
Reviewed-on: https://go-review.googlesource.com/25120
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
2 files changed