Track BUILDSYS_SOURCES_DIR for rebuilds in buildsys

[webern]

          nit: it feels like a (pre-existing) mistake to not track `BUILDSYS_SOURCES_DIR` if we are tracking `BUILDSYS_PACKAGES_DIR` - if we override the build to point to a completely different sources tree then that should invalidate any builds that used those sources.

Originally posted by @bcressey in #134 (comment)