Eolian allows non-public members in interfaces and mixins (usually
@protected). As both kinds are converted to C# interfaces, this
causes problem as non-public members are forbidden in C# interfaces.
This commit changes eolian_mono by removing those members from the C#
interfaces. If a generated class implements the interface, the method is
generated as if it were a protected member of the class directly.
With this, C# devs won't be able to directly implement protected Eo
methods from interfaces. (But this really does not make sense from the
C# point of view).
Depends on D9799