Rewrite MockAgent to talk directly to mock over Unix socket

[mizdebsk]

MockAgent implements support for mock pm_request plugin that allows XMvn to communicate with DNF package manager to request installation of missing artifacts on-demand.
XMvn is typically ran in a chroot, while DNF is outsides of chroot. Communication between XMvn and DNF is done using Unix domain socket.

MockAgent currently calls Python script from javapackages that communicates over Unix socket. With JEP 380: Unix-Domain Socket Channels (Java >= 16) it should be possible for XMvn to use Unix domain sockets directly, allowing the bits from javapackages to be deprecated and eventually removed.

This feature should be implemented in a separate project. It can't be implemented directly in XMvn because we want to keep compatibility with Java 8 for now, see #196.

[mizdebsk]

This may be unneeded after all, as long as #312 is implemented

[mizdebsk]

Current plan is that XMvn 4 will keep MockAgent as-is -- calling Python script from javapacakges.
In XMvn 5, MockAgent will talk directly to mock via UNIX socket, using Java 16 APIs.