(Original problem discovered by Rahul Biswas)

Now that the packages for external products are on GitHub, it would be nice for them to contain files like the near-standard "README.md". Unfortunately, eupspkg.sh's default_prep command explicitly inspects tarball-and-patch (TaP) packages to make sure there are no other files in the package root directory. I think this is a little excessive.

There's a bypass if the environment variable TAP_PACKAGE is set, but the package's table file has not yet been setup, so it's not possible to set this variable there. (The comments for the default_prep command suggest that the dependencies have been setup, but it's not clear that this is necessarily the case if invoked by lsst-build.)

I suggest that we check instead for a ./.tap_package file to indicate definitively that this is a TaP package.