## Summary
I noticed that the diagnostic range in specific unpacking assignments is
wrong. For this example
```py
a, b = 1
```
we previously got (see first commit):
```
error: lint:not-iterable
--> /src/mdtest_snippet.py:1:1
|
1 | a, b = 1
| ^^^^ Object of type `Literal[1]` is not iterable
|
```
and with this change, we get:
```
error: lint:not-iterable
--> /src/mdtest_snippet.py:1:8
|
1 | a, b = 1
| ^ Object of type `Literal[1]` is not iterable
|
```
## Test Plan
New snapshot tests.
This causes the diagnostic to highlight the actual unresovable import
instead of the entire `from ... import ...` statement.
While we're here, we expand the test coverage to cover all of the
possible ways that an `import` or a `from ... import` can fail.
Some considerations:
* The first commit in this PR adds a regression test for the current
behavior.
* This creates a new `mdtest/diagnostics` directory. Are folks cool
with this? I guess the idea is to put tests more devoted to diagnostics
than semantics in this directory. (Although I'm guessing there will
be some overlap.)
Fixes#15866
