The go parser previously checked for invalid import paths, go/build,
seeing the parse error would put files with invalid import paths into
InvalidGoFiles. golang.org/cl/424855 removed that check from the
parser, which meant files with invalid import paths not have any parse
errors on them and not be put into InvalidGoFiles. Do a check for
invalid import paths in go/build soon after parsing so we can make
sure files with invalid import paths go into InvalidGoFiles.

This fixes an issue where the Go command assumed that if a file wasn't
invalid it had non empty import paths, leading to a panic.

Fixes #60754
Updates #60230
Updates #60686

Change-Id: I33c1dc9304649536834939cef7c689940236ee20
Reviewed-on: https://go-review.googlesource.com/c/go/+/502615
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
(cherry picked from commit 962753b)
Reviewed-on: https://go-review.googlesource.com/c/go/+/502697