glib requires G_LOG_DOMAIN defined so that log messages are labeled
to belong to NetworkManager or libnm.

However, we don't actually want to use glib logging. Our library libnm
MUST not log anything, because it spams the user's stdout/stderr.
Instead, a library must report notable events via its API. Note that
there is also LIBNM_CLIENT_DEBUG to explicitly enable debug logging,
but that doesn't use glib logging either.

Also, the daemon does not use glib logging instead it logs to syslog.
When run with `--debug`.

Hence, it's not useful for us to define different G_LOG_DOMAIN per
library/application, because none of our libraries/applications should
use glib logging.

It also gets slightly confusing, because we have the static library like
`src/libnm-core-impl`, which is both linked into `libnm` (the library)
and `NetworkManager` (the daemon). Which logging domain should they use?

Set the G_LOG_DOMAIN to "nm" everywhere. But no longer do it via `-D`
arguments to the compiler.

See-also: https://developer.gnome.org/glib/stable/glib-Message-Logging.html#G-LOG-DOMAIN:CAPS

"libnm-core/" is rather complicated. It provides a static library that
is linked into libnm.so and NetworkManager. It also contains public
headers (like "nm-setting.h") which are part of public libnm API.

Then we have helper libraries ("libnm-core/nm-libnm-core-*/") which
only rely on public API of libnm-core, but are themself static
libraries that can be used by anybody who uses libnm-core. And
"libnm-core/nm-libnm-core-intern" is used by libnm-core itself.

Move "libnm-core/" to "src/". But also split it in different
directories so that they have a clearer purpose.

The goal is to have a flat directory hierarchy. The "src/libnm-core*/"
directories correspond to the different modules (static libraries and set
of headers that we have). We have different kinds of such modules because
of how we combine various code together. The directory layout now reflects
this.