cmd/guru: fix guessImportPath for Windows
cmd/guru: fix guessImportPath for Windows

Import paths in Go use forward slashes, not the operating system's path separator.

Fixes golang/go#24699.
Fixes golang/go#23318.

Change-Id: I190224f6b7236e8ee19893f3589dd9686cbf116a
GitHub-Last-Rev: 751b29ca5f2e52ee1124c4baca6e45eecf4a3052
GitHub-Pull-Request: golang/tools#56
Reviewed-on: https://go-review.googlesource.com/c/151057
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools