support wildcard imports in justfiles

In order to create re-usable toplevel justfiles, I started to split them up per command/tool in a .just-subfolder

For the moment I need to import them file by file and hardcoded each filename:

```
import '.just/git.just'
import '.just/poetry.just'
import '.just/precommit.just'
...
...

I prefer to import all files via wildcard:
```
import '.just/*.just'

...

The issue concerns imports in justfiles and a .just-subfolder; start by locating the existing single-file import handling and its tests. The work is done when an import such as `.just/*.just` discovers and loads the matching justfiles without requiring each filename to be listed.

Written by the indexing model from the issue text.