gh-117953: Split Up _PyImport_LoadDynamicModuleWithSpec()

[ericsnowcurrently]

I've pulled this out of #118116.

Basically, I've turned most of _PyImport_LoadDynamicModuleWithSpec() into two new functions (_PyImport_GetModInitFunc() and _PyImport_RunModInitFunc()) and moved the rest of it out into _imp_create_dynamic_impl(). There shouldn't be any changes in behavior.

This change makes some future changes simpler. This is particularly relevant to potentially calling each module init function in the main interpreter first. Thus the critical part of the PR is the addition of _PyImport_RunModInitFunc(), which is strictly focused on running the init func and validating the result. A later PR will take it a step farther by capturing error information rather than raising exceptions.

FWIW, this change also helps readers by clarifying a bit more about what happens when an extension/builtin module is imported.

• Issue: Single-Phase Init Extension Module Init Functions Still Run in Isolated Interpreters #117953

[ericsnowcurrently]

@encukou, if you have a few minutes, I'm mostly looking for a sanity check on this PR.

[encukou]

This PR looks good, but see my comment here: #118193 (comment)

An indirect test of a multiphase module with m_slots=NULL triggers the //assert that's commented out here.

[ericsnowcurrently]

Thanks. I'll take a look at that.

[encukou]

Thanks!
+1, I think this is good to merge. The is_singlephase thing can be sorted out later, see #117953 (comment)