cmd/go/internal/imports.ScanDir extracts a list of imports from a
directory. It's used instead of go/build.ImportDir when constructing
the build list. GOOS and GOARCH may be used to filter files.

With this change, imports.MatchFile understands that when the
"android" tag is set, the "linux" tag is implied.

Fixes #30888

Change-Id: Ia29bd1590b69c9183ab14a879d5fc1b639f8eaef
Reviewed-on: https://go-review.googlesource.com/c/go/+/168378
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>