[nikhilkumarpanigrahi]

Fixes #2872

I checked the legacy npm "license": "LicenseRef-LICENSE" convention on current develop.
It already works: proprietary-license_720.RULE (added in 936b3a4) has
referenced_filenames: LICENSE, so the license in the sibling LICENSE file is followed.
This works for the string, dict and list forms, and for bower.json.

The LicenseRef-LICENSE.md variant did not work. LicenseRef-LICENSE.md is listed in
other_spdx_license_keys of proprietary-license, so it was matched by the
auto-generated SPDX identifier rule. That rule has no referenced_filenames, so
LICENSE.md was never followed and the package was reported as proprietary-license.

Changes

• Add proprietary-license_1064.RULE for LicenseRef-LICENSE.md with
  referenced_filenames: LICENSE.md, same as proprietary-license_720.RULE.
• Add regression tests in tests/packagedcode/test_license_detection.py for both
  LicenseRef-LICENSE and LicenseRef-LICENSE.md in a package.json, with an MIT
  license in the sibling file.
• Add a CHANGELOG entry.

Results

package.json license	Sibling file	Before	After
LicenseRef-LICENSE	LICENSE (MIT)	mit	mit
LicenseRef-LICENSE.md	LICENSE.md (MIT)	proprietary-license	mit
LicenseRef-LICENSE	none	proprietary-license	proprietary-license

Tasks

• Reviewed contribution guidelines
• PR is descriptively titled 📑 and links the original issue above 🔗
• Tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR
• Commits are in uniquely-named feature branch and has no merge conflicts 📁
• Updated documentation pages (if applicable)
• Updated CHANGELOG.rst (if applicable)

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[nikhilkumarpanigrahi]

@AyanSinhaMahapatra this PR is ready for review.

The license_validate_ignorables_2 failure is not related to this PR. It is the same pre-existing failure on fsl-1.1_1.RULE and fsl-1.1_2.RULE (from #5121), which are missing ignorable_urls: https://fsl.software/FSL-1.1-ALv2.template.md.

See also #5382 and #5364.