hugolib: Enable override of theme base template only

hugolib: Enable override of theme base template only

This commit fixes the base template lookup order to match the behaviour of regular templates.

```
1. <current-path>/<template-name>-baseof.<suffix>, e.g. list-baseof.<suffix>.
2. <current-path>/baseof.<suffix>
3. _default/<template-name>-baseof.<suffix>, e.g. list-baseof.<suffix>.
4. _default/baseof.<suffix>

For each of the steps above, it will first look in the project, then, if theme is set,
in the theme's layouts folder.
```

Fixes #2783