Allow general requirements as PyPISpec inputs

[nicholasjng]

Requirements are processed as dictated by packaging.requirements.Requirement.

In essence, this also allows auditing a pinned requirements.txt file as generated e.g. by uv pip compile.

Caveat: Only PyPI URLs are audited, no custom indexes / git dependencies are supported.

---

A CLI test on this branch:

$ abi3audit -v "google-benchmark>=1.8.0,<2"
[10:41:20] 💁 google-benchmark>=1.8.0,<2: 5 extensions scanned; 0 ABI version mismatches and 0 ABI violations found 

Requirement errors are given as hints to the processing error:

$ abi3audit -v "google-benchm?rk>=1.8.0,<2"
[10:42:51] 👎 processing error: 'google-benchm?rk>=1.8.0,<2' does not look like a valid wheel, shared object, or package name                                                   
           hint: Expected end or semicolon (after name and no valid version specifier)                                                                                          
               google-benchm?rk>=1.8.0,<2                                                                                                                                       
                            ^ 

[woodruffw]

Thanks!