pipreqs should not be case sensitive

We should handle all package names in lowercase, since **pip is case insensitive**.
- see PEP-426 (https://peps.python.org/pep-0426/#name)
- see PEP-508 (https://peps.python.org/pep-0508/#names)

This will also reduce the amount of mappings. Many of them only match the names to upper-/lowercase.

Start by tracing how pipreqs reads imported package names and applies its package mappings. Use the PEP-426 and PEP-508 name rules as the reference, then verify that mixed-case package names are normalized consistently and redundant case-only mappings are no longer needed.

Written by the indexing model from the issue text.