bup: add support for "internal" subcommands

bup: add support for "internal" subcommands

Look for the existing something-cmd.py commands as both
lib/cmd/bup-something and lib/bup/cmd/something.py, and don't load the
latter as a module unless specifically allowed, so that we can create
separate rename commits as we convert each relevant external command
to an internal command.  This should make it easier to see what
actually changed in the git history, e.g. with --ignore-space-change.

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