In order for users to be able to inspect the feature level of Signed
References in a repository, have `rad inspect --sigrefs` attempt to
load Signed References, and, if successful, print the feature level.
The `help` module is no longer needed, since `clap` handles that.
This meant that the re-exports of each command's `ABOUT` could be removed and
the `const`s can now be private.
Lorenz Leutgeb
Fintan Halpenny
Richard Levitte
Matthias Beyer