cmd/go/internal/vgo: ignore appengine imports

As an unfortunate special case, just ignore import of appengine or appengine/*
when scanning a source tree to find modules that need to be imported.
A more aggressive option would be to ignore all paths that don't start
with an element with a dot (that is, don't start with a plausible domain name),
but I don't want to require that all modules have a dot at the start of the name.

The real problem here is that appengine was introduced so long ago that it
predates "go get" and did not use URL-like paths, and some code exists that
is still using those old imports. I am unaware of any other special cases like
that. If they arise, we can add them too, or we can revisit whether there is a
more general fix. For now this should suffice.

Fixes golang/go#25881.

Change-Id: I38cdbc1cd83a2f55d87f598bd0cbf1f6bcade88a
Reviewed-on: https://go-review.googlesource.com/118757
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ross Light <light@google.com>