[build-script] Refactor out BuildScriptInvocation into its own file and out of the top level build-script file.

This shrinks the number of lines in build-script by 50% and just makes it easier
to read. It will also enable me to refactor parts of it into pieces without
touching the main build-script file. This is really library code... build-script
itself doesn't care about how BuildScriptInvocation is actually implemented
under the hood.

Author: gottesmm