debian/control: Fix the dependencies.

  * Fix up the Build-Depends.  The Python requirements for the
    architecture-independent packages are quite heavy, so split out
    Build-Depends-Indep.

  * Add detailed version information to the various build and runtime
    dependencies.

  * Observe that `tripe-keys' depends on curl(1) to run, so we need it
    at build time for testing, and the `tripe-keys' package needs it at
    runtime.