[Refactor] Rename Lint and LintGroup's is_loaded to is_externally_loaded 
          #124522
        
          
      
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The field being named
is_loadedwas very confusing. Turns out it's true for lints that are registered by external tools like Clippy (I had to look at #116412 to know what the variable meant). So I renamedis_loadedtois_externally_loadedand added some docs.