Provide a mechanism to fetch all workspace (PDE) bundle pools

[merks]

PDE maintains a bundle pool in <workspace>/.metadata/.plugins/org.eclipse.pde.core/.bundle_pool. In org.eclipse.pde.internal.core.target.P2TargetUtils.findWorkspaceRepos(Set<URI>) it tries to find such bundle pools in other workspaces by looking at the preferences. Because Oomph maintains a resource with the locations of all installations and all workspace (and their associations) in <user.home>/.eclipse/org.eclipse.oomph.setup/setups/locations.setup this approach can be generalized. Also, because Tycho can benefit from this information, providing a utility in p2, org.eclipse.equinox.internal.p2.repository.helpers.RepositoryHelper.getWorkspaceBundlePools(), analogous to RepositoryHelper.getSharedBundlePools() would be beneficial.

See also #373