purl-next: Pub - on demand, data collection

[TG1999]

https://pub.dev/api/packages/flutter

[DennisClark]

should get merged soon

[TG1999]

This is done now

Reference:

• Add collector for pub ecosystem #712

Steps to test:

• To test this install purldb and set up using README and run make run. This will start server at http://127.0.0.1:8001/.
• Make a request to http://127.0.0.1:8001/api/collect/?purl=pkg:pub/flutter@0.0.1
• You will get a response like this

[
    {
        "url": "http://127.0.0.1:8001/api/packages/99049c00-ca8d-4bf2-a3ff-4d013baa2429/",
        "uuid": "99049c00-ca8d-4bf2-a3ff-4d013baa2429",
        "filename": "0.0.1.tar.gz",
        "package_sets": [],
        "package_content": "source_archive",
        "purl": "pkg:pub/flutter@0.0.1",
        "type": "pub",
        "namespace": "",
        "name": "flutter",
        "version": "0.0.1",
        "qualifiers": "",
        "subpath": "",
        "primary_language": null,
        "description": "A framework for writing Flutter applications",
        "release_date": null,
        "parties": [],
        "keywords": [],
        "homepage_url": "http://flutter.io/",
        "download_url": "https://pub.dev/packages/flutter/versions/0.0.1.tar.gz",
        "bug_tracking_url": null,
        "code_view_url": null,
        "vcs_url": null,
        "repository_homepage_url": null,
        "repository_download_url": null,
        "api_data_url": null,
        "size": null,
        "md5": null,
        "sha1": null,
        "sha256": null,
        "sha512": null,
        "copyright": null,
        "holder": null,
        "declared_license_expression": null,
        "declared_license_expression_spdx": null,
        "license_detections": [],
        "other_license_expression": null,
        "other_license_expression_spdx": null,
        "other_license_detections": [],
        "extracted_license_statement": "[]",
        "notice_text": null,
        "source_packages": [],
        "extra_data": {},
        "package_uid": "pkg:pub/flutter@0.0.1?uuid=99049c00-ca8d-4bf2-a3ff-4d013baa2429",
        "datasource_id": null,
        "file_references": [],
        "dependencies": [],
        "resources": "http://127.0.0.1:8001/api/packages/99049c00-ca8d-4bf2-a3ff-4d013baa2429/resources/",
        "history": "http://127.0.0.1:8001/api/packages/99049c00-ca8d-4bf2-a3ff-4d013baa2429/history/"
    }
]