`spack pkg list` tests were broken by #29593 for cases when your `builtin.mock` repo
still has stale backup files (or, really, stale directories) sitting around. This
happens if you switch branches a lot. In this case, things like this were causing
erroneous packages in the mock listing:
```
var/spack/repos/builtin.mock/packages/
foo/
package.py~
```
- [x] make `list_packages` consider only directories with one-deep `package.py` files.
a0d4630448