lua: implement chained searchers

This patch continues the refactoring of Tarantool package.loaders
required for Tarantool integrity protection.

Considering the way "app" paths injection is implemented in Tarantool
loaders to save the loading priority, the chained loaders machinery
introduced in the commit 7e9051c4 ("lua:
don't set built-in modules to package.loaded") is also implemented for
the new searching mechanism. The approach is the same as the one used
for chaining loaders. As a result, "app" paths searchers are injected
into the "cwd" paths searchers the same way it has been done in the
original loaders context.

This patch also introduces numerical indices into <package.searchers>
analogue to provide the convenient searchers <-> loaders mapping used in
the final patch of this series.

Needed for tarantool/tarantool-ee#585

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring