Debian PURLs fail to resolve in ScanCode.io / FetchCode

While working with ScanCode.io and FetchCode over the past week, I ran into an issue with Debian PURL resolution

Debian PURLs (e.g. pkg:deb/debian/bash@5.2.15) consistently fail to resolve.
This happens both locally and on a ScanCode.io server instance.

**To Reproduce in Scancode.io**
1. Open ScanCode.io
2. Create a new project
3. In Inputs, add: "pkg:deb/debian/bash@5.2.15"
4. select scan_single_package pipeline
5. Click Create

**Result**
The pipeline fails during download_missing_inputs.

**Error screenshots**

**In Fetchcode**
In WSL Activate your venv in the fetchcode directory

In Python

> from fetchcode import fetch
> fetch("pkg:deb/debian/bash@5.2.15")

Traceback (most recent call last):
File "", line 1, in
File "/home/vikas/fetchcode/src/fetchcode/__init__.py", line 148, in fetch
url, scheme = get_resolved_url(url, scheme)
File "/home/vikas/fetchcode/src/fetchcode/__init__.py", line 117, in get_resolved_url
url, scheme = resolution_handler(url)
File "/home/vikas/fetchcode/src/fetchcode/__init__.py", line 128, in resolve_url_from_purl
raise ValueError("Could not resolve PURL to a valid URL.")
ValueError: Could not resolve PURL to a valid URL.

> from fetchcode.download_urls import download_url
> download_url("pkg:pypi/requests@2.31.0")
'https://files.pythonhosted.org/.../requests-2.31.0.tar.gz'
> download_url("pkg:deb/debian/bash@5.2.15")
(No Output)

FetchCode cannot resolve Debian PURLs
Failure happens at PURL → URL resolution

**Observations**
FetchCode currently does not resolve Debian PURLs to concrete .deb download URLs
ScanCode.io relies on this resolution during download_missing_inputs
As a result, Debian PURLs fail early in the pipeline
Debian packages commonly appear in SBOMs, so this limits coverage

**Related context**
There are prior discussions and partially related work around Debian support in this repository,
#89 but based on current behavior, Debian PURL resolution is still incomplete or not fully wired through FetchCode → ScanCode.io.

**Propsed Approach**
I'll start by Identifying the appropriate Debian metadata sources and then adding support to fetch .deb URLs.