Incorrect resolution of nested modules

[dcreager]

In #14825 I tried creating a test case that refers to a class in a nested module:

```py path=a/b.py
class C: ...
```

```py
import a.b
x: type[a.b.C]
```

This produced an unexpected unresolved-attribute error:

Type <module 'System("/src/a/b.py")'> has no attribute b

This suggests that our module resolution code is binding a to a.b, and not to a as the Python spec requires.

[MichaReiser]

@carljm this shouldn't be too hard to fix? Maybe something for a contributor to take on?

[carljm]

@MichaReiser I suggested @dcreager might take a look, good intro task for the module resolver.

[AlexWaygood]

Great spot @dcreager! There is an existing TODO in the codebase regarding submodule imports, but this doesn't seem to be the same issue; I think it is a previously unknown bug.

For the record, the existing TODO is here:

ruff/crates/red_knot_python_semantic/src/types/infer.rs

Lines 2155 to 2163 in 269e47b

	// TODO:
	// - Absolute `*` imports (`from collections import *`)
	// - Relative `*` imports (`from ...foo import *`)
	// - Submodule imports (`from collections import abc`,
	// where `abc` is a submodule of the `collections` package)
	//
	// For the last item, see the currently skipped tests
	// `follow_relative_import_bare_to_module()` and
	// `follow_nonexistent_import_bare_to_module()`.

The tests referenced by that TODO don't exist anymore, however; that's because they've been ported to mdtests. They're now here:

ruff/crates/red_knot_python_semantic/resources/mdtest/import/relative.md

Lines 114 to 143 in 269e47b

	## Bare to module
	```py path=package/__init__.py
	```
	```py path=package/foo.py
	X = 42
	```
	```py path=package/bar.py
	# TODO: support submodule imports
	from . import foo # error: [unresolved-import]
	y = foo.X
	# TODO: should be `Literal[42]`
	reveal_type(y) # revealed: Unknown
	```
	## Non-existent + bare to module
	```py path=package/__init__.py
	```
	```py path=package/bar.py
	# TODO: support submodule imports
	from . import foo # error: [unresolved-import]
	reveal_type(foo) # revealed: Unknown
	```

[dcreager]

Fixed in #14946 and #15026