cmd/go: fix mod_install_pkg_version

mainPackagesOnly now includes non-main packages matched by literal
arguments in the returned slice, since their errors must be reported.

GoFilesPackages attaches the same error to its package if
opts.MainOnly is true. This changes the error output of 'go run'
slightly, but it seems like an imporovement.

For #42088

Change-Id: I8f2942470383af5d4c9763022bc94338f5314b07
Reviewed-on: https://go-review.googlesource.com/c/go/+/310829


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>