Fix ArgumentExtractor implementation bug

[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?
cc @tomerd @abertelrud

[Kyle-Ye]

@swift-ci please test

[Kyle-Ye]

/// Extracts options of the form --<name> <value> or --<name>=<value> from the remaining arguments, and returns the extracted values.

Due to the implementation issue of ArgumentExtractor, currently we can not use --<name>=<value> form successfully as the comments says cc @ethan-kusters

[abertelrud]

Thank you very much for the fix @Kyle-Ye! Yes, it should definitely be nominated for 5.7. The bar is probably quite high for 5.6.2 as I understand it.

[abertelrud]

@swift-ci please smoke test

[Kyle-Ye]

Swift 5.6 #4289
Swift 5.7 #4288

[Kyle-Ye]

@swift-ci please smoke test