Resolve license using LICENSE.md file (like NPM does)

[Vadorequest]

Currently, if there is no package.json:license field, then this plugin will not resolve the license even if a LICENSE.md file exists. See jestjs/jest#8734

[greyscaled]

I just ran into a problem whereby the package arg was flagged as having an invalid license. I assume git-legal failed to manage a case where there is both a LICENSE.md and a license entry in the package.json.

Here's the output of arg from node_modules

$ cat LICENSE.md 
MIT License

Copyright (c) 2017-2019 Zeit, Inc.

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

$ cat package.json
{
  "name": "arg",
  "version": "4.1.3",
  "description": "Another simple argument parser",
  "main": "index.js",
  "types": "index.d.ts",
  "repository": "zeit/arg",
  "author": "Josh Junon <junon@zeit.co>",
  "license": "MIT",
  "files": [
    "index.js",
    "index.d.ts"
  ],
  "scripts": {
    "pretest": "xo",
    "test": "WARN_EXIT=1 jest --coverage -w 2"
  },
  "xo": {
    "rules": {
      "complexity": 0,
      "max-depth": 0,
      "no-div-regex": 0
    }
  },
  "devDependencies": {
    "chai": "^4.1.1",
    "jest": "^20.0.4",
    "xo": "^0.18.2"
  }
}