Require a relative sls file

Ref. #5098

It looks like the require statement takes an absolute path to the required sls file. It would be cool if it could also support relative sls files, e.g.:

```
include:
  - .local

somename:
  - somemodule:somefunction
      - require:
          sls: .local
```

Today (0.17.5) the last require statement would have to read `sls: path.to.here.local`
