go/gcimporter15: interpret relative srcDir relative to cwd
go/gcimporter15: interpret relative srcDir relative to cwd

This is a backport of the respective changes in golang.org/cl/19393.

For golang/go#14215.

Change-Id: I8d60dd6daa827a60597f3af925e6732914537319
Reviewed-on: https://go-review.googlesource.com/19394
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

golang/tools,cmars/tools,syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,cmars/tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,cmars/tools,syreclabs/go-tools,golang/tools,golang/tools,cmars/tools