tools: ynl: move python code to separate sub-directory

tools: ynl: move python code to separate sub-directory

Move python code to a separate directory so it can be
packaged as a python module. Updates existing references
in selftests and docs.

Also rename ynl-gen-[c|rst] to ynl_gen_[c|rst], avoid
dashes as these prevent easy imports for entrypoints.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/a4151bad0e6984e7164d395125ce87fd2e048bf1.1736343575.git.jstancek@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>