Enabling this flag for a `home.file` entry causes the target to be
unconditionally overwritten. The option is not visible in
documentation for now and shouldn't be relied on for general use.
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`.
