Fortran 2008 [1] adds support for a new syntax related to modules:

    submodule ( ParentModule ) SubModule
    submodule ( ParentModule : SubModule ) NestedSubModule

Both of these mean that the current source file requires the module
`ParentModule` to be available if it is not provided in the current
file.  Teach our Fortran dependency scanner to parse this syntax to
extract this relationship.  For now simply tolerate the nested submodule
case and extract only the dependency it expresses on the main module.
Further work will be needed to extract dependencies among nested
submodules.

[1] http://fortranwiki.org/fortran/show/Fortran+2008

Closes: #16234