lua: implement searchers for preload and builtin

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

Since Tarantool follows Lua 5.1[1] semantics, the set of routines in
package.loaders is used to tweak Lua modules loading. E.g. overriding
machinery has been implemented on top of the package.loaders.
Unfortunately, package.loaders are not as flexible as they should be and
this mechanism is redesigned in Lua 5.2[2].

Starting from Lua 5.2 package.loaders has been superseded by
package.searchers. This is quite a natural change, since nobody wants to
tweak the "loading" rules, but rather the "searching" process. As a
result of this redesign evolution up to the Lua 5.4[3], users are able
to obtain the "source" of the module being loaded (i.e. file name or
specific sentinel for preload packages).

This patch introduces Tarantool internal table with package.searchers
and reimplements package.preload and tarantool.builtin loaders using the
machinery described above:
* The new searcher function, respecting Lua 5.4 semantics, is
  implemented for each loader.
* Each searcher is wrapped with the function to implement "dummy" loader
  respecting Lua 5.1 semantics.

[1]: https://www.lua.org/manual/5.1/manual.html
[2]: https://www.lua.org/manual/5.2/manual.html
[3]: https://www.lua.org/manual/5.4/manual.html

Needed for tarantool/tarantool-ee#585

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring