This imports some of the basic support classes from the Perl Mojolicious
distribution for use in kdesrc-build.

We don't use it for much currently but it will enable us to replace much
of the current kdesrc-build custom modules with higher-performing and
better-supported code. It will also permit features that have been
requested for awhile.

In particular, will replace:

* IPC stuff for async (in favor of Mojo::IOLoop->subprocess)
* Exception classes (in favor of Mojo::Exception)
* Much of ksb::Util is replaced by Mojo::Util
* Weird custom logging (in favor of Mojo::Log)
* File templating (in favor of Mojo::Template and Mojo::Loader)

and will add:

* Useful co-routines (someday, assuming we can make Future::AsyncAwait a
  dependency... and even if not, we can use continuation-passing style
  now).
* Useful Web clients/user-agent (for pulling information from Bugzilla
  and Gitlab)
* Useful Web serving (to permit kdesrc-build to be API introspected by a
  GUI client such as a Plasmoid...