`free-unknown` should be dismissed as license clues when there are other proper detections in the file

[chinyeungli]

This plugin was created in [Realytics](https://www.realytics.io/) in 2017. Thank you for supporting Open Source.

## License

MIT License

License detection yeilds free-unknown AND mit for the above text.

The free-unknown is from

"license_expression": "free-unknown",
"license_expression_spdx": "LicenseRef-scancode-free-unknown",
"from_file": "code/fork-ts-checker-webpack-plugin-6.5.3.tgz-extract/package/README.md",
"start_line": 434,
"end_line": 436,
"matcher": "2-aho",
"score": 50,
"matched_length": 3,
"match_coverage": 100,
"rule_relevance": 50,
"rule_identifier": "free-unknown_88.RULE",
"rule_url": "https://github.com/nexB/scancode-toolkit/tree/develop/src/licensedcode/data/rules/free-unknown_88.RULE",
"matched_text": "Open Source.\n\n## License",
"matched_text_diagnostics": "Open Source.\n\n## License"

Another sample:
busybox-1.37.0/networking/ssl_helper/ssl_helper.c

    - score: '100.0'
      matcher: 2-aho
      end_line: 8
      rule_url: https://github.com/nexB/scancode-toolkit/tree/develop/src/licensedcode/data/rules/free-unknown_7.RULE
      from_file:
      start_line: 8
      matched_text: ' * This software is open source; you can redistribute it and/or modify'
      match_coverage: '100.0'
      matched_length: 5
      rule_relevance: 100
      rule_identifier: free-unknown_7.RULE
      license_expression: free-unknown
      license_expression_spdx: LicenseRef-scancode-free-unknown
    - score: '94.06'
      matcher: 3-seq
      end_line: 19
      rule_url: https://github.com/nexB/scancode-toolkit/tree/develop/src/licensedcode/data/rules/gpl-2.0-plus_573.RULE
      from_file:
      start_line: 8
      matched_text: |
         * This software is open source; you can redistribute it and/or modify
         * it under the terms of the GNU General Public License as published by
         * the Free Software Foundation; either version 2 of the License, or
         * (at your option) any later version.
         *
         * This program is distributed in WITHOUT ANY WARRANTY; without even the
         * implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
         * See the GNU General Public License for more details.
         *
         * You should have received a copy of the GNU General Public License
         * along with this program; if not, write to the Free Software
         * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
      match_coverage: '94.06'
      matched_length: 95
      rule_relevance: 100
      rule_identifier: gpl-2.0-plus_573.RULE
      license_expression: gpl-2.0-plus
      license_expression_spdx: GPL-2.0-or-later

The free-unknown is detected from the same starting line of the detected gpl-2.0-plus license notice.

It'll be great if it can remove the free-unknown from the detection to prevent noise.