Remove CLI pkg_resources dependency

[siddicky]

Summary

• replace CLI metadata and plugin discovery calls with importlib.metadata
• use the importlib-metadata backport below Python 3.8
• support both legacy group mappings and modern selectable entry points
• add regression coverage for isolated CLI startup, version output, and legacy plugin discovery

This is a smaller alternative to #248 that removes the pkg_resources fallback entirely, following the backport suggestion in that PR.

Fixes #252.
Related: #243, #245, #246, #248.

Verification

• uv run --isolated --python 3.14 --with . python -m unittest tests.test_cli: 3 tests passed
• basedpyright tests/test_cli.py: 0 errors, 0 warnings
• uv build: sdist and wheel built successfully
• clean wheel install with uv/Python 3.14: shodan --help and shodan version both exit 0
• production source contains no pkg_resources references

The existing live-API suite still has unrelated failures caused by stale Exploits/host fixtures and changed account/filter behavior; all new CLI tests pass.