Use a replacement for boost::filesystem::path

See https://svn.boost.org/trac/boost/ticket/8708.  Also, since we
don't need most of what that class provides, we limit risk by serving
ourselves exactly the functionality we need.
Remove dead code
Temporarily hardcode ignoring of "/CVSROOT/" paths and adjust to
leading slash expectations

Eventually we'll make path matching more robust in patrie, and
canonicalize on no-leading-or-trailing-slash paths, and work out a
scheme to explicitly ignore paths in the ruleset.