This change makes use of the `extend` function inside `lib` to inject
a new `hm` field containing the Home Manager library functions. This
simplifies use of the Home Manager library in the modules and reduces
the risk of accidental infinite recursion.
PR #994
In particular support source files whose name start with `.` or
contain characters not allowed in the nix store, such as spaces.
Also add some test cases for `home.file`.
