Common file for all plugins?

### Enhancement Type

Improve an existing feature

### Describe the enhancement

Currently the plugins downloaded from various sites are provided in separate environment variables. Some also support providing them in a text file.

I feel it'd make it a lot cleaner if one could configure a mods text file that would allow managing mods across multiple websites (e.g. by expecting a `spiget:`/`modrinth:`/... prefix before each mod). Along with comments it would probably make managing mods way less of a chore.

No files, tests, or entry points are named. Start by locating the existing plugin environment-variable and text-file handling, then define how one shared file handles site prefixes and comments. Done means mods from multiple sites can be managed through that file without separate configuration variables.

Written by the indexing model from the issue text.