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_completion/src/tests.rs

@@ -10,6 +10,7 @@ mod items;
 mod pattern;
 mod type_pos;
 mod predicate;
+mod sourcegen;
 
 use std::mem;