Wrong purl to url conversion for debian

In the spec, https://github.com/package-url/purl-spec there is an example of Debian PURL:
```
pkg:deb/debian/curl@7.50.3-1?arch=i386&distro=jessie
```

packageurl-python converts this PURL in the following URL:
```
https://deb.debian.org/debian/pool/main/c/curl/curl_7.50.3-1_i386.deb
```
This gives:
```
The requested URL was not found on this server.
```
