Support Swift package manager source layout conventions

Make it easy for a build author to create a build that uses the Swift PM source layout conventions.

This might be by making these conventions the default, or by adding a plugin or some DSL element to allow the layout convention to be declared.

As an increment, we might first add a way for the build author to declare the source files/directories for a Swift project in a single place and have all compilation tasks and Xcode project generation honour this, rather than having to configure this in several places.

No files, tests, or entry points are identified in the issue, so first map the build and Xcode project-generation areas that configure Swift source locations. Compare the existing configuration with Swift Package Manager source layout conventions, then define a focused approach whose completion can be verified by compilation tasks and generated Xcode projects honoring one declared source layout.

Written by the indexing model from the issue text.