Missing metadata attributes in certain file formats

### Bug Description

Certain file formats (especially ancillary file formats such as apt Packages, git protocol responses, snap API responses, or pip simple index files) may be missing important metadata fields such as artefact name and vendor.

### To Reproduce

Download artefacts and check the session report on ancillary files.

### Relevant log output

```shell
"metadata": {
"type": "application/x.canonical.snap-info",
"sha1": "05d8611ee864a73e96dda816942458ce03649dd0",
"sha256": "751dcb74e02bcd15a045bb4a87ae9fa90ecfc3085e68c40d5c5e52f59bc91e80",
"size": 4364,
"name": "",
"version": "",
"vendor": "",
"description": "",
"author": "",
"author-email": "",
"architecture": "",
"license": "",
"copyright": ""
}
```
```

Start by reproducing the issue with ancillary files such as apt Packages, git protocol responses, snap API responses, and pip simple index files, then inspect their session reports. Compare the metadata extraction for these formats with the expected artefact name, vendor, and other fields. Done means the affected reports contain the available metadata instead of empty values.

Written by the indexing model from the issue text.