Path normalization for containerized builds

For compiler caching to work properly, paths to source files need to be able to match across builds. That means things like the ceph version, git commit id, etc. should not be present in any path components.

Containerized builds will help greatly with this, and are a [work-in-progress](https://github.com/ceph/ceph-build/tree/pipeline-container): CentOS builds appear to properly get a 100% cache hit rate on rebuilds. Ubuntu builds, however, do not. See [this build of the WIP pipeline](https://jenkins.ceph.com/job/ceph-dev-pipeline/177/pipeline-graph/), particularly how the CentOS branch takes 30min to compile while the Ubuntu branch takes 73min.

We need to investigate how to normalize paths for Ubuntu and Debian builds.