Update comment in ListPackages()
Update comment in ListPackages()

The comment indicated that ListPackages() could handle multiple ignored
`package main` files in another package. Thanks to how how `go/build`
works, this wasn't true. This commit adds an extra bit to the comment
noting as much.

ibrasho-forks/dep,golang/dep,ibrasho-forks/dep,golang/dep,golang/dep