Fix experimental-api-diff command and add a test

[owenv]

This fixes swift package experimental-api-diff, which doesn't need or accept library search path arguments. I also added a small test to avoid regressions in the short term. The motivation here is to begin working towards semantic versioning suggestions for packages when tagging a release.

[owenv]

@swift-ci please smoke test

[owenv]

Hmm, I might need to skip the test in some environments - Xcode toolchains presumably won't have swift-api-digester until sometime after swiftlang/swift#36381 is merged

[owenv]

@swift-ci please smoke test

[owenv]

14:35:25 [504/850] Testing CommandsTests.PackageToolTests/testAPIDiff

Looks like in the full smoke test config on macOS we are able to run the test

[owenv]

I added a second commit here which:

• Loads the modules in the current version of the package directly instead of dumping JSON and comparing that against the baseline
• Removes the invert-baseline option since there are better ways of detecting semver minor changes and it will be difficult to support under the new model

[owenv]

@swift-ci please smoke test

[owenv]

@swift-ci please test self hosted Linux

[owenv]

@swift-ci please smoke test Linux

[owenv]

@swift-ci please smoke test

[tomerd]

thanks for your patience @owenv. 5.5 was just branched this weekend so we are finally ready to take this, @neonichu can work with you to land