Support reporting unknown SPDX LicenseRef as their own license

[pombredanne]

This is split from #2502 (comment)

In there @soimkim wrote

I also have a similar issue.
I want to print the items marked with SPDX License, but the ScanCode json result is output as follows.

• Information written to the file:
  # SPDX-License-Identifier: LicenseRef-Sample-Proprietary

• ScanCode json result :

"licenses": [
        {
          "key": "unknown-spdx",
          "score": 83.33,
          "name": "Unknown SPDX license detected but not recognized",
          "short_name": "unknown SPDX",
          "category": "Unstated License",
          "is_exception": false,
          "owner": "Unspecified",
          "homepage_url": null,
          "text_url": "",
          "reference_url": "https://scancode-licensedb.aboutcode.org/unknown-spdx",
          "scancode_text_url": "https://github.com/nexB/scancode-toolkit/tree/develop/src/licensedcode/data/licenses/unknown-spdx.LICENSE",
          "scancode_data_url": "https://github.com/nexB/scancode-toolkit/tree/develop/src/licensedcode/data/licenses/unknown-spdx.yml",
          "spdx_license_key": "LicenseRef-scancode-unknown-spdx",
          "spdx_url": "https://github.com/nexB/scancode-toolkit/tree/develop/src/licensedcode/data/licenses/unknown-spdx.LICENSE",
          "start_line": 3,
          "end_line": 3,
          "matched_rule": {
            "identifier": "spdx-license-identifier: unknown-spdx",
            "license_expression": "unknown-spdx",
            "licenses": [
              "unknown-spdx"
            ],

Although it is not a license registered in SPDX, is there a way to print the License ID ("Sample-Proprietary" in the example above) if it is written according to the notation of the SPDX License?

If "key" extracted as "unknown-spdx", how about outputting "identifier" in "matched_rule" as License ID written?

[pombredanne]

@AyanSinhaMahapatra ping.
We need to accommodate for any licenseref from anyone
There a few possible cases:

1. This is a known, stable ref for which we have as a license

(eventually all such refs would end up there) and we have the ref as SPDX license key

2. This is not known:
   2.1 We can find the corresponding license text for instance with REUSE conventions
   2.2 Or this is in some SPDX document

We should assign the detected license on the corresponding file or SPDX blurb to this ref.