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.
PUP-2040/PE-3113 Remove symlink-ignore tests for symlinkless OSes

Before this commit, the spec tests checked for symlink-ignore behavior on all
OSes. After this commit, the spec tests only check symlink-ignore
behavior on OSes that have symlinks.
Merge pull request #3018 from andersonmills/fix/master/PUP2040-fix_symlink_testing_on_windows

PUP-2040/PE-3113 Remove symlink-ignore tests for symlinkless OSes