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
Given an inner type, the former function generates a type that expect
DAG option values. The latter function is only present to temporarily
allow the `programs.ssh.matchBlocks` to keep accepting list values.
Because `extraPackages` and `overrides` expect functions as values it
has not been possible to perform merges. This adds suitable types for
these options that allow reasonable merging.
