imports: print dir of candidates in addition to import path

The import path is ambiguous in the presence of vendoring (e.g. golang/go#20610)

Change-Id: I22f372b233b8554e3d9210b383a7df7a6a0f3eee
Reviewed-on: https://go-review.googlesource.com/53470
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>