Better fix for https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/732

The original issue was that when compiling NetworkManager with slibtool
the create-exports-NetworkManager.sh script gave the NetworkManager-all-sym
slibtool wrapper script to nm(1) instead of the actual binary. This is
because slibtool and GNU libtool do not place the compiled programs in
the same location. The original fix was to test both locations, but this
is bit of a hack especially since the build system should not be using the
.libs directory directly. However with $(LIBTOOL) --mode=execute this is
not a problem since both implementations can figure out where the correct
binary is.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/805

Like with "libnm-core/", split "libnm/" into different directories for
the public headers, for the implementation and for the helper "aux"
library.

"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.

"shared/nm-meta-setting.[hc]" contains meta data about settings.
As such it is similarly used by libnm-core (as internal API) and
by clients (as extension of public API of libnm). However, it must
be compiled twice, because while it defines in both cases a
NMMetaSettingInfo type, these types are different between internal and
public API.
Hence, the files must also be compiled twice (and differently), once
against libnm-core and once against the client helper library.

Previously, the file was under "shared/", but there it's a bit odd
it doesn't clearly belong anywhere.

There are two goals here:

 - copy the file to the two places where it is used. We also have
   a "check-tree" unit test that ensures those files don't diverge in
   the future.

 - we no longer require CFLAGS set during built. Instead, the sources
   should control the build. For that we have new (simple) headers
   "nm-meta-setting-base.h" that define the right behavior for the
   impl files.

There is still an ugliness (among several): the files must be named the
same for libnm-core and clients/common. Preferably, all our sources have
unique names, but that is not possible with this scheme (without
introducing other ugliness). To mitigate that, include the files only at
one exact place.

Our source tree also has certain consistency requirements. Since the
source is in git, this is a rather static check. However, we want to
ensure that future changes don't break it by adding a test.