Add Arch Linux .SRCINFO parser

[karthiknew07]

Fixes #4406

Description

Adds support for parsing Arch Linux .SRCINFO files generated by makepkg. These files contain package metadata for the Arch User Repository (AUR) and Arch Linux packages.

Changes

• Added SrcinfoHandler in src/packagedcode/srcinfo.py
• Added test suite in tests/packagedcode/test_srcinfo.py
• Added 5 test data files covering various scenarios

Features

• Parses pkgbase (global) and pkgname (per-package) sections
• Extracts name, version, description, license, dependencies
• Handles architecture-specific fields (e.g., depends_x86_64)
• Supports split packages (multiple pkgname sections)
• Stores checksums and sources in extra_data

Testing

• ✅ All 5 tests passing locally
• ✅ Tested with real Arch Linux package examples
• ✅ Handles edge cases (split packages, arch-specific deps)

Tasks

• [x] Reviewed contribution guidelines
• [x] PR is descriptively titled and links issue #4406
• [x] Tests pass locally (5/5 passing)
• [x] Commits are in uniquely-named feature branch (add-srcinfo-support)
• [x] No merge conflicts
• [ ] Updated documentation pages (N/A - new parser)
• [ ] Updated CHANGELOG.rst (will update if requested)

References

• .SRCINFO spec: https://wiki.archlinux.org/title/.SRCINFO
• Format: Simple key=value pairs in sections