compiler: remove "DIR/../" when joining relative import path

Otherwise if DIR does not exist, the path does not work. This matches
what the gc cmd/compile tool does, because it calls path.Join.

The test for this is the cmd/go tests, to be added in a follow-up CL.

Change-Id: I27486f3795fb78a1ada1730c4e375268acf4ff65
Reviewed-on: https://go-review.googlesource.com/45691
Reviewed-by: Than McIntosh <thanm@google.com>
1 file changed