Rollup merge of #124522 - blyxyas:refactor-is-loaded, r=jieyouxu

[Refactor] Rename `Lint` and `LintGroup`'s `is_loaded` to `is_externally_loaded`

The field being named `is_loaded` was very confusing. Turns out it's true for lints that are registered by external tools like Clippy (I had to look at rust-lang/rust#116412 to know what the variable meant). So I renamed `is_loaded` to `is_externally_loaded` and added some docs.