internal: overhaul code generation

* Keep codegen adjacent to the relevant crates.
* Remove codgen deps from xtask, speeding-up from-source installation.

This regresses the release process a bit, as it now needs to run the
tests (and, by extension, compile the code).

crates/ide_assists/src/tests.rs

@@ -1,3 +1,4 @@
+mod sourcegen;
 mod generated;
 
 use expect_test::expect;