This commit separates profile management (setting profile and creating
GC root) from file management (removing and adding managed files
within the user's home directory).
This is a step towards deprecating profile management within the
activation script, instead relying on the caller of the activation
script managing the profile.
This introduces some rudimentary integration tests using the NixOS
test framework. The intent is to better catch regressions when doing
more elaborate changes that may affect overall Home Manager behavior.
Note, the tests are currently not run automatically.
