Compile all of the plugins at the start of a build with swiftbuild

[cmcgee1024]

When building with the swiftbuild build system there can be a compilation problem with the
plugin.swift with one of the plugins. However, you won't discover that unless you run the
command plugin specifically. This is not the behaviour of the existing native build system
that does a plugin pass at the start of a build.

Implement a similar plugin pass with the swiftbuild system to catch problems with the plugin
code when building the packages.

Fixes #8977

[cmcgee1024]

@swift-ci please test

[jakepetroules]

nit: classes should be final by default if you're not using inheritance

[cmcgee1024]

This class is declared within the scope of this function, so I think that would limit the ability for anything external to extend it. Is it still worth declaring it final in this case?

[dschaefer2]

The next question then is, why is this a class and not a value type if the lifetime is so short.

[cmcgee1024]

@swift-ci please test

[cmcgee1024]

@swift-ci test Windows

[bkhouri]

suggestion: instead of an if block, can we use a switch block , and explicitly call out we don't expect any output from SwiftBuild? e.g.:

switch data.buildSystem {
   case .native:
        #expect(!stdout.contains("Compiling plugin MyBuildToolPlugin"), "stderr: \(stderr)")
        #expect(stdout.contains("Compiling plugin MyCommandPlugin"), "stderr: \(stderr)")
    case .swiftbuild:
       // nothing specific
       break
     case .xcode:
         Issue.record("Test expected have not been considered")
}