Refactor Installer::locateExecutableInDefaultPaths() into a separate
utility class, ExecutableFinder. This class is already used in plenty of
places outside of the installer, so it's ripe for being extracted.
This class is located in utils/ due to the dependency upon
Shell::command(). Once that no longer has a dependence upon MediaWiki,
this class can be moved to libs/ too.
Change-Id: I175465acc0d64f990445ce05fabcee8b88a0b259
