CLI: F*: interfaces flag is a bit too expressive

[W95Psp]

hax into fstar --interfaces and hax into --include_namespaces accept the same kind of value (they share a same type and parser).

With the latter, the user can either:

• exclude a namespace (prefix -);
• include an item and all its local dependencies (prefix +);
• include just an item without including its dependencies (prefix +!).

The former accepts the same grammar, but +! makes no sense: either we extract an interface for a module, or we don't, but there is no third option.

Here, either the Rust type InclusionClause should be made polymorphic to allow or not +!, or we should have a different type.

[W95Psp]

This will be closed when #465 is done