Improve PIFBuilder testability by exposing PackagePIFBuilder objects

[pmattos]

Motivation:

Improve PIFBuilder testability by exposing all intermediary PackagePIFBuilder objects created during PIF construction.

For instance, this something we need to be able to add the required tests for PR #9436, where a fix was made to how PackagePIFBuilder computes some PIF metadata not directly used by SwiftPM (only by Xcode, and potentially other external clients too).

Modifications:

This PR extracts the following intermediary function from the existing — and very long! — PIFBuilder.constructPIF operation:

package func makePIFBuilders(
    buildParameters: BuildParameters
) async throws -> [(ResolvedPackage, PackagePIFBuilder, any PackagePIFBuilder.BuildDelegate)] 

...allowing it to be unit tested too! No new features are actually added — just plain refactoring :-)

Result:

Improved testability in SwiftBuildSupport.PIFBuilder class.