Instead of touching pool/<pkgname>/.building, move it to building/<pkgname>

This avoids a race condition when cleaning from child and master is looking
for work in pool/. The child cleanup removes the .building and then the
pool/<pkgname> dir while the master then sees pool/<pkgname> is empty
and ready to build, so it picks it to build again. By moving the pkgname
to building/ instead, fully atomic operations can be done, avoiding
the need for lockf (coming next)