Usage example:

```lua
local loaders = require('internal.loaders')
local vshard = loaders.require_first('vshard-ee', 'vshard')
```

The function is for internal use.

It would be nice to have something of this kind in the public API, but
I'm not going to solve it within this patch.

Needed for https://github.com/tarantool/tarantool-ee/issues/815

NO_DOC=no public API changes
NO_CHANGELOG=see NO_DOC