lua: implement common legacy loader generator

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

Though all Lua 5.1 loaders machinery will be superseded by Lua 5.4
searchers machinery, Tarantool still should follow Lua 5.1 semantics, so
the "legacy" loader is required. This patch implements the common helper
<gen_legacy_loader> to wrap all loaders and make the new machinery
conform the Lua 5.1 semantics.

Moreover, since all tweaks and helpers are implemented for searching
machinery, <chain_loaders> is not required anymore.

Needed for tarantool/tarantool-ee#585

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring