cmd/go: add functions to read index file

cmd/go: add functions to read index file

The data read is used for three primary functions: ImportPackage,
IsDirWithGoFiles and ScanDir. Functions are also provided to get this
information from the intermediate package representation to cache
the information from reads for non-indexed packages.

Change-Id: I5eed629bb0d6ee5b88ab706d06b074475004c081
Reviewed-on: https://go-review.googlesource.com/c/go/+/403975
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>