Refactor prebuilt to use srcs supplier function

Previously, prebuilt had two ways to provide the src value, as a
pointer to an []string and using reflection. That meant that any code
that needed to check the src to use had to understand the two ways of
accessing it.

This change refactors the code to abstract the origin of the src values
being a supplier function. That insulates the callers from having to
know the details and makes it easy to add new suppliers in future.

Test: m nothing
Bug: 142918168
Change-Id: I5cc7517c46cce96924c9f34ba1b046b468f7684a