Add content addressed mode for URL SCM mirrors

[jkloetzke]

Is your feature request related to a problem? Please describe. With preMirrorPrepend, the recommended configuration options describe how to setup a local mirror of downloaded sources. In #673 it became clear that the recommended configuration was prone to false sharing and subsequent build errors.

Now the full URL is used but this might not be optimal. If the source URL is overridden (e.g. in a corporate environment behind a proxy), but the content hash stays the same, the mirror URL is affected as well, leading to duplicate downloads.

Describe the solution you'd like Add a content-addressable mirror mode. We mirror only deterministic packages anyway.