The godot base derivation was implemented by passing a recursive
attrset to mkDerivation. This is problematic because recursive
references to attributes that are later overridden don't notice the
override. Switched to the approach of giving mkDerivation a lambda
that receives a self argument, which allows recursive references to
the final value after overrides are applied.
The related derivations (for export templates, headless, and server
builds of godot) duplicated content from the base derivation. This
refactor eliminated that by using overridable attributes to
parameterize the scripts.
The main motivation for this refactor was to help me add derivations
for mono builds of godot.
Renamed to godot3 to distinguish from version 4, which is a complete
rewrite and effectively a different tool altogether.
