metadata: compile dependencies only when the package is selected

metadata: compile dependencies only when the package is selected

This is intended to reduce build time for situations like the following
where python and python-six and their dependencies could still be built
as long as any subpackage within the srcpackage was selected regardless
of the selection state of openvswitch-python

    define Package/openvswitch-python
      ...
      DEPENDS:=+python +python-six
    endef

Previously we work around this by specifying the dependency as
+PACKAGE_openvswitch-python:python, which is unintuitive

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>