cmd/go/internal/modindex: ignore non-source files for index

cmd/go/internal/modindex: ignore non-source files for index

We were saving non-go file information in the module index files,
leading in an unnecessary increase in memory usage in modules
containing many non-go files. This was a bug because this information
is never used. Don't save that information.

For #54226

Change-Id: I0644064f83f96e3a9f43b7e66ca94d69d9603376
Reviewed-on: https://go-review.googlesource.com/c/go/+/439118
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>