Make Hadrian flavours more compositional

Here we implement the idea floated in #18942 (closed) by introducing a notion of flavour transformers, accessible via the command-line.

This makes the --flavour option accept the following syntax:

flavour = flavour_name ('+' transformer_name)*

which allows the user to modify the flavour without modifying UserSettings.hs.

We currently provide flavour transformers which cover most of the existing configuration options that are commonly changed.