Allow to use only modules from direct dependencies

[awvwgk]

Currently all modules of all dependencies are visible to all projects in the dependency tree. Depending by accident on a module of a transient dependency is not an error but could break the build if the dependency tree changes and the module or the transient dependency is removed upstream. Selectively hiding/including modules would also provide a mechanism to cope with name clashes for unrelated modules with the same name.

Requires #355

[LKedward]

Selectively hiding/including modules would also provide a mechanism to cope with name clashes for unrelated modules with the same name.

How would this work? Surely you could still end up with symbol clashes?

[awvwgk]

Unless the two modules don't specify the same function names there won't be a symbol clash.

One mechanism could be to have separate module directories for each subproject like in this tree:

build
└── gfortran_2A42023B310FA28D
    ├── dep-1
    │   └── lib
    │       └── wrapper.mod
    ├── dep-1-dep
    │   └── lib
    │       └── utils.mod
    ├── dep-2
    │   └── lib
    │       └── m_dep.mod
    └── main-proj
        ├── app
        │   ├── main-exe
        │   │   └── utils.mod
        │   └── helper-exe
        │       └── utils.mod
        └── lib
            └── main.mod

I think this is mainly useful for executable, test or example targets where multiple targets might define modules with the same name. But could also be used to hide dependencies with problematic module names behind a simple reexport wrapper.