Fix release archives and add tests

Previously, an archive created by "git archive" would fail to build,
because the versioning code was broken (in part by
03d35993b87f49753542e705e37949a46964be17) for any build outside a git
clone.  Although "bup version" may not really have been quite right
since the switch from "bup-VERSION" to "VERSION release tags.

Fix the problem by reintroducing the content of the older
_version.py.pre as a permanent _release.py file, which (as with the
previous incarnation) is a template that's expanded during "git archive"
via export-subst (cf. gitattributes(5)).

Add a version module as the public interface to versioning, and have it
sort out whether or not the current tree is an official release.  Adjust
configure-version to manage _checkout.py instead of _version.py (drop
_version.py), so that the version module is effectively just
disambiguating between _release.py and _checkout.py.

Adjust "bup version" to use the new version module and to handle
unprefixed VERSION tags.

Finally, add test-release-archive.sh, invoked via "make distcheck", so
we (hopefully) don't unknowingly break this again.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>