makefiles: move extra deps into new MOD vars

To make the makefiles more similar.

That is, add the following new variables:

* MOD_HDRS
* MOD_SRCS
* MOD_OBJS

And move existing module-specific header and object dependencies into
`MOD_HDRS` and `MOD_OBJS`, respectively.  `MOD_SRCS` is added mostly for
symmetry/consistency.

Note: Use `MOD_` as a prefix instead of `EXTRA_` to avoid confusion, as
the latter is currently used for global variables (such as
`EXTRA_CFLAGS`), as opposed to module-specific variables.

Note2: Add them directly into the HDRS/SRCS/OBJS variables to avoid
cluttering the existing recipes with an extra variables unnecessarily.
This also allows, for example, referencing all of the object
dependencies with `$<` if `$(OBJS)` is the first dependency (at least in
GNU make).

Note3: Since HDRS/SRCS/OBJS use simple assignment (through `:=`), the
MOD variables should appear before including src/common.mk (or
src/so.mk).

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