Using this function it is possible to make `home.file` create a
symlink to a path outside the Nix store. For example, a Home Manager
configuration containing
home.file."foo".source = config.lib.file.mkOutOfStoreSymlink ./bar;
would upon activation create a symlink `~/foo` that points to the
absolute path of the `bar` file relative the configuration file.
PR #1211
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`.