file loader escape hatches should test basename

[jasongrout]

#3743 should have tested against the basename of the file, rather than just the start of the string in the webpack configs. That would allow for relative imports, etc.

Something like

      { test: p => path.basename(p).startsWith('JUPYTERLAB_RAW_LOADER_'), use: 'raw-loader' },
      { test: p => path.basename(p).startsWith('JUPYTERLAB_URL_LOADER_'), use: 'url-loader?limit=10000' },
      { test: p => path.basename(p).startsWith('JUPYTERLAB_FILE_LOADER_'), use: 'file-loader' },

[jasongrout]

I'm getting tired of these huge prefixes, though. Perhaps better would be some metadata in the package.json signifying which files should be loaded with raw, url, or file loaders, like jupyterlab.loaders.raw = ['list', 'of', 'files'].