WapmSource: config option to prioritize webc v3

[theduke]

Extend the BuiltinPackageLoader with a config option to prioritize webc v3 over v2.

We need this to gain confidence in v3 in different testing environments before we make v3 the default.

How to modify the source:

• fetch both v2 and v3 from the backend
• use a config option to prioritize either over the other

Example query:

query {
 getPackage(name:"xxx/yyy"){
  versions {
    version 
    piritaManifest
    manifest
    v2: distribution(version:V2) {
      webcSha256Hash
      webcDownloadUrl
      
    }
    
    v3: distribution(version:V3) {
      webcSha256Hash
      webcDownloadUrl
      
    }
  }
}
}