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.
SI-8375 saner binary incompat errors for macros

Inspired by Brian McKenna's RC1 migration experience, this commit improves
macro impl binding validation in order to provide more helpful diagnostic
for this quite frequent class of errors.
Addresses pull request feedback
Merge pull request #3603 from xeno-by/ticket/8364

SI-8364 fixes cxTree lookup for imports
Merge pull request #3606 from xeno-by/ticket/8375

SI-8375 saner binary incompat errors for macros