Reinstate 'initialBuildSteps' function

This patch reinstates the 'initialBuildSteps' function, as it is
used by stack in its implementation of the multi-repl feature.

A deprecation warning has been added to that function: calling it does
not suffice to prepare the sources for a package, as there are other
steps that one might also need to perform:

  - running pre-processors (such as alex/happy)
  - running pre-build hooks or custom logic (in build-type: Hooks
    or build-type: Custom or Configure)

Consumers wanting to prepare the sources of a package, e.g. in order to
launch a REPL session, are advised to run
`Setup repl --repl-multi-file=<fn>` instead.

Fixes #9856