plpython: Streamline initialization

plpython: Streamline initialization

The initialization of PL/Python (the Python interpreter, the global
state, the plpy module) was arranged confusingly across different
functions with unclear and confusing boundaries.  For example,
PLy_init_interp() said "Initialize the Python interpreter ..." but it
didn't actually do this, and PLy_init_plpy() said "initialize plpy
module" but it didn't do that either.  After this change, all the
global initialization is called directly from _PG_init(), and the plpy
module initialization is all called from its registered initialization
function PyInit_plpy().

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Matheus Alcantara <matheusssilv97@gmail.com>
Reviewed-by: li carol <carol.li2025@outlook.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Discussion: https://www.postgresql.org/message-id/f31333f1-fbb7-4098-b209-bf2d71fbd4f3%40eisentraut.org