Add "is_referenced" to files that were referenced (such as LICENSE)

## Short Description

When scanning sometimes files are referenced, such as "see file LICENSE in this source tree."

The JSON output then adds the scan results for the file `LICENSE` as well (which is great) but it doesn't indicate that this was a file that was referenced. Adding something like

```
"is_referenced": "yes"
```

would make it a lot easier to find files that were referenced in the scan results, and also very easy to spot when inspecting the results, or to highlight in a UI.

## Possible Labels

- new feature

## Select Category

- [X] Enhancement
- [ ] Add License/Copyright
- [X] Scan Feature
- [ ] Packaging
- [ ] Documentation
- [ ] Expand Support
- [ ] Other

## **Describe the Update**

An example is the following output from a scan of BusyBox:

```
"matches": [
{
"license_expression": "gpl-2.0-plus",
"license_expression_spdx": "GPL-2.0-or-later",
"from_file": "busybox-1.37.0/editors/awk.c",
"start_line": 7,
"end_line": 7,
"matcher": "2-aho",
"score": 100.0,
"matched_length": 12,
"match_coverage": 100.0,
"rule_relevance": 100,
"rule_identifier": "gpl-2.0-plus_544.RULE",
"rule_url": "https://github.com/nexB/scancode-toolkit/tree/develop/src/licensedcode/data/rules/gpl-2.0-plus_544.RULE"
},
```

This rule has a list of `referenced_filenames`, for which the scan results are included:

```
{
"license_expression": "gpl-2.0",
"license_expression_spdx": "GPL-2.0-only",
"from_file": "busybox-1.37.0/LICENSE",
"start_line": 1,
"end_line": 6,
"matcher": "2-aho",
"score": 100.0,
"matched_length": 45,
"match_coverage": 100.0,
"rule_relevance": 100,
"rule_identifier": "gpl-2.0_only2.RULE",
"rule_url": "https://github.com/nexB/scancode-toolkit/tree/develop/src/licensedcode/data/rules/gpl-2.0_only2.RULE"
},
{
"license_expression": "gpl-2.0",
"license_expression_spdx": "GPL-2.0-only",
"from_file": "busybox-1.37.0/LICENSE",
"start_line": 9,
"end_line": 348,
"matcher": "2-aho",
"score": 100.0,
"matched_length": 2931,
"match_coverage": 100.0,
"rule_relevance": 100,
"rule_identifier": "gpl-2.0_1022.RULE",
"rule_url": "https://github.com/nexB/scancode-toolkit/tree/develop/src/licensedcode/data/rules/gpl-2.0_1022.RULE"
}
],
```

When trying to find out what the referenced files are, I would first have to walk all the matches, extract `from_file`, check if this matches the `path` and if not, then it is a referenced file name. I would rather see something like a tag that says `is_referenced` or something similar, as that makes it easier to walk the results, plus it is a lot more logical from a programming point of view ("walk the results, report all the file that have `is_referenced`"). Even a simple `grep` (which I use often on scancode results) would now be possible.

## **How This Feature will help you/your organization**

Quicker filtering and processing of interesting results.

## **Possible Solution/Implementation Details**

Add `is_referenced` or something like it should be a straightforward solution.

## **Example/Links if Any**

## **Can you help with this Feature**