Fix swift package plugin --list

[neonichu]

Printing the list needs to continue beyond the first non-command plugin.

resolves #6804

[neonichu]

@swift-ci please smoke test

[tomerd]

add test?

[neonichu]

add test?

I guess that would make sense, but I am reluctant to add more integration tests here. We should refactor this entire thing so it becomes testable.

[neonichu]

I am going to try if making SwiftTool take a file system parameter may help here. It's still a bit too broad for my taste, but might at least enable us to write tests that aren't as slow and unmaintainable as the current command tests.

[neonichu]

I am going to try if making SwiftTool take a file system parameter may help here. It's still a bit too broad for my taste, but might at least enable us to write tests that aren't as slow and unmaintainable as the current command tests.

This works, but the test is still extremely slow (~8s when using a very basic package).

[neonichu]

It seems like the slowness is mostly the result of excessive and preemptive shelling out. I wonder if we could come to a model where we would instead do that on demand. In any case, I don't think I can solve that here and I'll add my test as-is.

[neonichu]

Ah, of course --list uses print directly :/

[neonichu]

Going to merge this as-is as discussed offline, #6827 will stay open to improve test infra and add test coverage for plugin --list.