refactor: name builtin vshard sources

It produces better error messages.

Before this commit:

```
tarantool> vshard.router.call()
---
- error: '[string "local log = require(''log'')..."]:575: attempt to
    compare number with nil'
...
```

With this commit:

```
- error: 'vshard/router/init.lua:575: attempt to compare number with
    nil'
```