[th/libnm-core-split] build: move "libnm-core/" to "src/" and split it

"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.