Extend eiqgen-based resolution with override modifiers

[ujhelyiz]

The eiqgen genmodel imports should be extended by control directives that change the precedence between EPackage registry packages and workspace genmodels.

Ideas from the meeting (subject to change):

• old default mode (before Change eiqgen-based resolution semantics #316): eiqgen overrides EPackage registry - shadowing is a warning/error
• default mode: eiqgen does not override EPackage registry . shadowing is an error
• override mode: eiqgen always overrides EPackage registry - shadowing is considered normal (no error/warning)
• flexible mode: eiqgen does not override EPackage registry - shadowing is considered normal

The concrete supported modes shall be discussed further before implementation.

[ujhelyiz]

Migrated to https://bugs.eclipse.org/bugs/show_bug.cgi?id=398821