Add more tests for import

[G. Allais]

• Import of f & implicit import of f (via transitive dependency) should not raise an "AmbiguousName" error when using f.

• Diamond imports (A -> B, A -> C, B -> D, C -> D) with conflicting definitions
  but not used in the main (=> should work just fine)

• Diamond imports & use conflicting definition (=> should raise "AmbiguousName")

• Imports in a mary page

[G. Allais]

Another one: ee90513