Don't have ./bup depend on main.py, etc.

Don't have ./bup depend on main.py, etc.

Instead create bup_deps (which includes bup) and have everything that
needs a functional ./bup depend on that set of targets.

Previously bup would often be rebuilt unnecessarily (even if it didn't
depend on other phony targets) because it depended on _version.py and
main.py.  Since bup ends up symlinked to the latter, and make looks at
symlink target timestamps, and _version.py would often be newer than
main.py, ./bup would be repeatedly rebuilt.

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