Use TextDomain as slug for single-file plugins

[apermo]

Summary

• For single-file plugins (no subdirectory), use the TextDomain header as the slug instead of the filename
• Fixes Hello Dolly (hello.php) being reported as "not found" — its TextDomain hello-dolly matches the wp.org slug, while the filename-derived hello does not
• Generic fix, not a Hello Dolly special case — works for any single-file plugin
• Plugins in subdirectories are unaffected (directory name is still used)

Fixes #78

Context

get_plugin_slug() derives the slug from the plugin's file path. For plugin-name/plugin-name.php, the directory name matches the wp.org slug. But single-file plugins like hello.php have no directory — the code strips the extension to get hello, which doesn't match the actual wp.org slug hello-dolly.

The TextDomain header reliably matches the wp.org slug for virtually all plugins. Using it as the slug source for single-file plugins resolves the mismatch.

Test in WordPress Playground

Hello Dolly is bundled with WordPress, so no extra plugins needed to reproduce.

Before (develop — Hello Dolly shows "not found"): Open in WordPress Playground

After (this branch — Hello Dolly resolved correctly): Open in WordPress Playground

Pre-installed test plugins: Apermo Admin Bar (outdated) — Hello Dolly is bundled with WordPress.

Test plan

• Open the Before link — Hello Dolly row shows "wordpress.org, plugin not found" and "No data available" everywhere
• Open the After link — Hello Dolly row shows "wordpress.org" with correct version, last update, tested up to, and rating
• Verify all other plugins still display correctly in the After link