Change source file paths to be relative to input

[willarmiros]

1. Add new attribute in JSON report for absolute_path, which provides the absolute path of the input path provided by the user. This will make it clearer what is being resolved if the user provides a relative path as the input_path. IMPORTANT: If the input_path is to a file, the absolute_path should be the absolute path to the directory that file is in.
2. In the source, scanned_files, and skipped_files fields, only reference file paths relative to the input path. For example, if the input path is /path/to/my/directory, and the directory has model1.pkl and model2.pkl, then output may look like this:

{
    "summary": {
        "total_issues_by_severity": {
            "LOW": 0,
            "MEDIUM": 0,
            "HIGH": 1,
            "CRITICAL": 0
        },
        "total_issues": 2,
        "input_path": "/tmp/test_model",
        "absolute_path": "/path/to/my/directory",
        "modelscan_version": "0.1",
        "timestamp": datetime.datetime.now(),
        "skipped": {
            "total_skipped": 0,
            "skipped_files": []
        },
        "scanned": {
            "total_scanned": 2,
            "scanned_files": ["model1.pkl", "model2.pkl"]
        }
    },
    "issues": [
        {
            "description": "Use of unsafe operator 'get' from module 'requests.api'",
            "operator": "get",
            "module": "requests.api",
            "source": "model1.pkl",
            "scanner": "pytorch",
            "severity": "HIGH"
        }
    ],
    "errors": []
}

If the input path is a file, then the source, scanned_files, and skipped_files fields can all just be the file name (no absolute path).