kbuild: unify modules(_install) for in-tree and external modules

[ Upstream commit 3e3005df ]

If you attempt to build or install modules ('make modules(_install)'
with CONFIG_MODULES disabled, you will get a clear error message, but
nothing for external module builds.

Factor out the modules and modules_install rules into the common part,
so you will get the same error message when you try to build external
modules with CONFIG_MODULES=n.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Stable-dep-of: c7b98de7 ("phy: qcom-qmp-combo: fix runtime suspend")
Signed-off-by: Sasha Levin <sashal@kernel.org>