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

Branch
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master

Details
-------
https://git.postgresql.org/pg/commitdiff/da265a87175be469db6c178b57862ad1fb26dd04

Modified Files
--------------
src/pl/plpython/plpy_main.c | 78 ++++++++++++++++++++-------------------
src/pl/plpython/plpy_plpymodule.c | 25 +------------
src/pl/plpython/plpy_plpymodule.h | 1 -
3 files changed, 41 insertions(+), 63 deletions(-)