Equinox p2 data paths

We need to stabilize the file system layout of p2 metadata so that we can self-host on p2-provisioned Eclipse installs, and use p2 to upgrade across builds.

Current State

• There is a p2 root directory, either supplied using the "eclipse.p2.data.area" system property, or by default it is eclipse/configuration/org.eclipse.equinox.p2.core/agentdata/.
• Under this root, we find:
  • install registry and profile registry
  • metadata/ subdirectory, containing the agent's metadata repository (AgentLocation.getMetadataRepositoryURL)
  • artifacts/ subdirectory, containing the agent's artifact repository (AgentLocation.getArtifactRepositoryURL). Also known as the download cache.
  • touchpoints/ subdirectory, containing:
    • director/ subdirectory, containing director's rollback repository
    • org.eclipse.equinox.p2.touchpoint.eclipse subdirectory
      • bundlepool/ subdirectory
        • plugins/ subdirectory containing the bundle pool.

Suggested approach

• In theory, everything that reads/writes data is pluggable. We want to be able to swap in another implementation of IInstallRegistry, IProfileRegistry, IDirector, IMetadataRepositoryManager, IArtifactRepositoryManager, Engine, etc, and have the other pieces continue to work.
• We should continue to have a single root directory defined by a system property, or by default it will be the *same* as the configuration location (not a sub-directory of configuration location).
• Each p2 plug-in must write into a sub-directory of that location, defined by their plugin namespace.
• There is no distinction between touchpoint data areas, and other data areas (for example the SimpleDirector's data area)
• We don't provide API to access the root, but we provide an API method in p2.core that dishes out the data location for a particular plugin namespace (AgentLocation.getDataLocation(String namespace))
• For example, the eclipse touchpoint would write its data under <root>/org.eclipse.equinox.p2.touchpoint.eclipse. Since the profile registry is currently implemented by the engine plugin, its file would be at <root>/org.eclipse.equinox.p2.engine/profileRegistry.xml.