The modules subsystem hides some low-level operations under API.
In particular the modules subsystem is responsible for

 - modules lookup in Lua's "package.search" storage
 - modules caching to eliminate expensive load procedure
 - function symbol resolving

Because naming is intersecting with current module functions
sitting in box/func, lets rename the later to schema_module
prefix. We will use this prefix in next patches to point the
modules in box.schema.func are just a particular user of
the general modules engine.

Part-of #4642

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>