go/build: don't add unparsable non-Go files to InvalidGoFiles

go/build: don't add unparsable non-Go files to InvalidGoFiles

go/build attempts to parse comments at the beginning of non-Go files
looking for //go:build or //+go build comments. Before this change,
if the beginning of the non-Go file failed to parse (perhaps because
it is in a format that isn't even meant to be built with Go code) the
file would be added to InvalidGoFiles. The comment for InvalidGoFiles
states that it contains Go files, so this is clearly incorrect
behavior.

Further, if there was a directory that only contained these unparsable
non-Go files, it would have a non-zero number of InvalidGoFiles, and
the matching code in cmd/go/internal/search/search.go in
(*Match).MatchDirs would treat it as a directory containing (invalid)
Go files and would match the directory as a Go package. This incorrect
behavior is also fixed by this CL.

Fixes #56509

Change-Id: Id0d905827c71f7927f7c2fa42b236181950af7e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/447357
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>