[5.6] Fix ArgumentExtractor implementation bug (#4287)

[Kyle-Ye]

Fix ArgumentExtractor implementation bug

Motivation:

swift-docc-plugin uses the PackagePlugin API and support generate-documentation action.

If we run

swift package --allow-writing-to-directory ./docs \
    generate-documentation --target SymbolKit --output-path ./docs

It will be fine.
However if we run

swift package --allow-writing-to-directory ./docs \
    generate-documentation --target=SymbolKit --output-path ./docs

It will loop infinitely.

And the core reason seems to be the ArgumentExtractor implementation in PackagePlugin Package.

Modifications:

• Fix the ArgumentExtractor implementation bug
• Add testExtractOption for ArgumentExtractor

Result:

When using --<name>=<value> form to specify the argument, the Plugin will no long loop infinitely.

Other consideration

I think this should be cherry-picked into release/5.7
But should it be cherry-picked into release/5.6.2?

[Kyle-Ye]

@swift-ci please smoke test

[Kyle-Ye]

Hello @tomerd (Release manager for release/5.6 swift-package-manager)

Does this meet the merge bar of 5.6.2?

If not, I'll just close it and only cherry-pick this into release/5.7.

[tomerd]

thanks @Kyle-Ye. will see if we can include it in the next possible window

[abertelrud]

Looks like this is already in main (via #4287) as well as release/5.7 (via #4288) so the only remaining question is whether it is a candidate for 5.6.