This separates extmod source files from `py.mk`. Previously, `py.mk`
assumed that every consumer of the py/ directory also wanted to include
extmod/. However, this is not the case. For example, building mpy-cross
uses py/ but doesn't need extmod/.
This commit moves all extmod-specific items from `py.mk` to `extmod.mk` and
explicitly includes `extmod.mk` in ports that use it.
Signed-off-by: David Lechner <david@pybricks.com>
c118b5d0e4