Accept a content.xml as input

[waynebeaton]

We should be able to map content in a p2 repository to ClearlyDefined identifiers.

For example, a content.xml file has entries like this:

...
<unit id='org.hamcrest.core' version='1.3.0.v201303031735' singleton='false'>
...

Which we can map to p2/orbit/p2.eclipse-plugin/org.hamcrest.core/1.3.0.v201303031735.

Note that the Eclipse Foundation's backend can handle these and has compatible mappings for many (perhaps most) Java libraries that we've reviewed. ClearlyDefined knows nothing of any of this.

The p2/orbit part is the convention that I've used for all content served via p2 repositories. Clearly, the orbit part isn't generally correct; so this may evolve as we learn more.