PUP-2040/PE-3113 Ignore symlinks in module build with pmtignore.

Before this commit, pmtignore functionality worked, but only after
checking for symlinks, so it was impossible to ignore symlinks. This
commit allows symlinks to be ignored, too.
Merge pull request #3014 from andersonmills/fix/master/PUP2040-gitignore_symlinks

PUP-2040/PE-3113 Ignore symlinks in module build with pmtignore/gitignore.