Tracker is an innocent bystander hindering libsoup3 port, since the
libsoup2/3->tracker->gtk3 dependency chain imposes a libsoup version,
graphical applications using libsoup cannot port at their own pace.

Make our remote code (connection & endpoint) be a private module
that is built against both versions of libsoup (if found), then we
pick one at runtime, with a preference on libsoup3 if libsoup2 .so
file is not seen in the already loaded libraries.

This patch should be reverted ASAP, once we can consider libsoup2
deprecated.

Fixes: https://gitlab.gnome.org/GNOME/tracker/-/issues/320