I apologize for not following closely earlier...   once packages have
been installed, what happens, if anything, to package-relative paths
and the code that uses them?  The installed layout would look
something like (still flexible):

/opt/
  ros/
    <xturtle>/
      include/
      lib/
      bin/
      etc/
      share/
        <package1>/
          cmake/
          resources/
            (stuff)
        <package2>/
          cmake/
          resources/
             ....

Who needs to know whether we are installed or not?  Where would the manifest go?

-t