go/internal/gcimporter: extract ChanDir and fake FileSet logic

go/internal/gcimporter: extract ChanDir and fake FileSet logic

This code will be useful for the indexed format importer, so break it
out to be easier to reuse separately.

Change-Id: Ie7e6b2ed89770e1ed9aa1edf11682fe35d6bb373
Reviewed-on: https://go-review.googlesource.com/107617
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>