asgen/backends: Initial solus backend

Example local configuration
```
"ArchiveRoot": "/var/lib/solbuild",
"Suites": {
    "local": {
        "sections": ["main"],
        "architectures": ["x86_64"]
    }
},
```
Index path: /var/lib/solbuild/local/eopkg-index.xml

Example remote configuration
```
"ArchiveRoot": "https://cdn.getsol.us/packages",
"Suites": {
    "slaney": {
        "sections": ["main"],
        "architectures": ["x86_64"]
    }
},
```
Index path: https://cdn.getsol.us/packages/slaney/eopkg-index.xml.xz

Author: joebonrichie