[ty] Refactor detection of import statements for completions

This commit essentially does away of all our old heuristic and piecemeal
code for detecting different kinds of import statements. Instead, we
offer one single state machine that does everything. This on its own
fixes a few bugs. For example, `import collections.abc, unico<CURSOR>`
would previously offer global scope completions instead of module
completions.

For the most part though, this commit is a refactoring that preserves
parity. In the next commit, we'll add support for completions on
relative imports.

Author: BurntSushi