Introduce dist/pythonlibs directory to store RIOT python packages.
This directory is exported via PYTHONPATH by the build system to
make it commonly available.
BUILDDEPS are files / make targets that should be build before compiling.
It can include packages source download, generating headers, modules.
It is the equivalent of `APPDEPS` but not limited to the application.
It cannot be done right now with `APPDEPS` as it is used in `BASELIBS` and
fixing it requires changing mips using it for source files.
Set the application module name with APPLICATION_MODULE.
The default module name is "application_$(APPLICATION)".
This should fix name conflicts where an application/test has the same name as
an existing module and so both write to the same archive.
