`buzzy doc` can show undocumented variables

It will say that there's no documentation, and print out the current
value, if it exists.
Set base path for repo and package environments

So that relative paths are handled correctly.  For repositories, the
base path is easy; it's whatever we set for the `repo.base_dir`
variable.  Right now we only create new package environments for
single-package filesystem repositories.  For those, the base path of the
package is the value of the repository's `repo.base_dir` variable.