SI-8364 fixes cxTree lookup for imports

This is reminiscent of the bug that I recently fixed in paradise:
https://github.com/scalamacros/paradise/commit/0dc4e35883d357b7cbcdfd83b5b4821c1dcc0bb1.

When doing something non-standard with contexts, we usually have to keep
in mind that new contexts are created not only for trees that demarcate
blocks of code, but also for imports.