Bundle built-in data sources as a GResource.

Bundle built-in data sources as a GResource.

This was primarily meant to aid our test suite, whose goal is to run
successfully without requiring evolution-data-server (or its data files)
to be installed.  The tests inadvertently depend on these built-in data
sources being present.

But this is good to do anyway.  Bundling the data sources directly in
the evolution-source-registry binary guarantees they're always present,
which the E-D-S APIs already assume, and avoids the usual problems with
installed data files: I/O errors, user meddling, etc.