fix: 🐛 remove package prefix in error message (#241)

# Description

The hardcoded prefix led to some issues when integrating
check-datapackage into sprout. See #240 for details.

Test with:

```sh
uv run python -c 'import check_datapackage as cdp; cdp.check({"resources": []}, error=True)'
```

Output before this PR:

```
1 issue was found in your `datapackage.json`:

At package.resources:
|
| resources: []
|            ^^
[] should be non-empty
```

Output after this PR:

```
1 issue was found in your `datapackage.json`:

At resources:
|
| resources: []
|            ^^
[] should be non-empty
```

I will say that with a top level path like this, it does appear a bit
redundant to list `resources` twice and the `package.resource`
aesthetically gave a bit more variation, but it is minor and for more
nested paths this will look good and be shorter.

Closes #240

Needs a quick review.

## Checklist

- [ ] Formatted Markdown
- [x] Ran `just run-all`

---------

Co-authored-by: Luke W. Johnston <lwjohnst86@users.noreply.github.com>

Author: joelostblom

src/check_datapackage/check.py

@@ -142,7 +142,7 @@ def _create_explanation(issue: Issue) -> str:
 
     number_of_carets = len(str(issue.instance))
     return (  # noqa: F401
-        f"At package{issue.jsonpath.removeprefix('$')}:\n"
+        f"At {issue.jsonpath.removeprefix('$.')}:\n"
         "|\n"
         f"| {property_name}{': ' if property_name else '  '}{issue.instance}\n"
         f"| {' ' * len(property_name)}  [red]{'^' * number_of_carets}[/red]\n"