Since commit v3.7.0-rc1~73^2~1 (Fortran: Add support for submodule
syntax in dependency scanning, 2016-09-05) we support parsing Fortran
sources that use submodule syntax, but it left addition of `.smod`
dependencies to future work.  Add it now.

The syntax

    submodule (module_name) submodule_name

means the current source requires `module_name.mod` and provides
`module_name@submodule_name.smod`.  The syntax

    submodule (module_name:submodule_name) nested_submodule_name

means the current source requires `module_name@submodule_name.smod`
provides `module_name@nested_submodule_name.smod`.

Fixes: #17017