Instead of creating libmm-plugin* and libmm-shared* libraries that are
dlopen()-ed on runtime, allow incorporating all plugins into the
daemon binary itself.
This makes the startup of the daemon much faster and also avoids
issues with builds that require linker namespace isolation.
Fixes https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/issues/674
We are going to allow including the plugin sources built within the
ModemManager daemon binary; moving the sources within the daemon
sources directory makes it easier.
The test service file is used in the test-plugin-generic unit tests,
which require the ModemManager daemon to be launched in a private test
DBus session.