Map optional extras to the source modules allowed to import them

[akul-ameya]

Optional extras under [project.optional-dependencies] are currently treated as regular dependencies. That means this is invisible to deptry:

[project]
dependencies = ["httpx"]

[project.optional-dependencies]
snowflake = ["snowflake-connector-python"]

mypackage/core.py

import snowflake.connector

Users who install mypackage without [snowflake] then fail at import time. Treating the extra as a dev group (optional_dependencies_dev_groups) is the wrong fix, because mypackage/snowflake.py is allowed to import snowflake.connector when the extra is installed. That import does not need to be ImportError-guarded.

A mapping from extra name to first-party modules would catch both "base imports an extra-only package" and "the postgres extra imports a snowflake-only package".

[akul-ameya]

I have been using a check for this and I have just raised a PR (#1664) incorporating that solution, so it can hopefully address it appropriately. I request someone to please review it.

It maps extras to first-party modules and reports extra-only imports outside those modules. It does not require optional imports to be ImportError-guarded.