rpm,deb: package always-enabled plugins in a separated package

rpm,deb: package always-enabled plugins in a separated package

ceph-mgr depends on it in the sense that these plugins are
a hard dependency of ceph-mgr. like cmake depends on cmake-data,
even if the later cannot function on its own without the former.

the reason why we need a separated package is that mgr modules
are written in python, and should be arch independent. this change
helps the downstream to understand this and help to minimize the
divergence between upstream and downstream, because distros like
debian and its derivatives encourage splitting architecture independent
data into its own package.
see
https://www.debian.org/doc/manuals/developers-reference/best-pkging-practices.html#architecture-independent-data

also, the python related runtime dependencies are also moved into
ceph-mgr-modules-core.

if a python dependency is shared by another ceph-mgr-* package and
ceph-mgr-modules-core. it's only added to ceph-mgr-modules-core for
simplicity and consistency. as the name of dependency might vary
on different distros, so it's tedious and error-prone to repeat
these conditons and checks.

Signed-off-by: Kefu Chai <kchai@redhat.com>