Add support for pkgbuild

Add support for pkgbuild. To use, add USE_PKGBUILD=1 to your Makefile (or override for everything in luggage.local), and ensure your package scripts are correctly named (preinstall/postinstall in the brave new pkgbuild world).

Also tweak location of luggage.local import, and ensure environment variables are preserved so $STAMP and other time-sensitive variables aren't re-evaluated in sub-makes.