Ok, I went ahead and did 'bzr remove debian/source' as well as removed the cached
tarballs and build-area. A subsequent 'bzr bd -S' gave me:
serge@sergelap:~/src/x$ bzr bd -S
Building using working tree
Running in merge mode
Looking for a way to retrieve the upstream tarball
Using apt to look for the upstream tarball.
apt could not find the needed tarball.
Trying to use get-orig-source to retrieve needed tarball.
make: *** No rule to make target `get-orig-source'. Stop.
Trying to run get-orig-source rule failed
No watch file to use to retrieve upstream tarball.
bzr: ERROR: Unable to find the needed upstream tarball: user-mounts_1.1.orig.tar.gz.
With debian/source/format set to