[6.4] Ensure targets merged into products by the build system can still be selected using --target

[owenv]

Closes #10275
Closes #9138

When generating PIF, a package target representing an executable or test target may be merged into a product. These remain targets in the user-facing model, and should be selectable using --target on the CLI. Update BuildSubset.pifTargetName to account for this case, fixup an existing test for executables, and add a new one for tests.

[owenv]

@swift-ci test

[bkhouri]

Looks good, though I have a few non-blocking comments related to the tests

[bkhouri]

suggestion: can we split the expectation to each build system validate the expected error message?

Suggested change

	#expect(error.stderr.contains("Could not find target named 'notarealtarget'") ||
	error.stderr.contains("no target named 'notarealtarget'"))
	let expectedErrorMessage = switch buildSystem {
	case .native:
	// to be updated with expected string
	"Could not find target named 'notarealtarget'"
	case .swiftbuild:
	// to be updated with expected string
	"no target named 'notarealtarget'"
	}
	#expect(error.stderr.contains(expectedErrorMessage)

[bkhouri]

suggestion: can we wrap this in a do{} block to ensure the two use cases will always be tested?

Suggested change

	let result = try await build(
	["--target", "exec2"],
	packagePath: fullPath,
	configuration: data.config,
	buildSystem: buildSystem,
	)
	switch buildSystem {
	case .native:
	#expect(result.binContents.contains("exec2.build"))
	#expect(!result.binContents.contains("exec1.build"))
	case .swiftbuild, .xcode:
	#expect(result.binContents.contains(executableName("exec2")))
	#expect(!result.binContents.contains(executableName("exec1")))
	}
	do {
	let result = try await build(
	["--target", "exec2"],
	packagePath: fullPath,
	configuration: data.config,
	buildSystem: buildSystem,
	)
	switch buildSystem {
	case .native:
	#expect(result.binContents.contains("exec2.build"))
	#expect(!result.binContents.contains("exec1.build"))
	case .swiftbuild, .xcode:
	#expect(result.binContents.contains(executableName("exec2")))
	#expect(!result.binContents.contains(executableName("exec1")))
	}
	}

[owenv]

I'll look at some of the test cleanups in a followup since they're comparatively minor