Reexported imports not tracked properly in fine-grained incremental mode

[msullivan]

We fail to propagate changes when the target of an import changes.

Here, we swap what module a imports foo from, but m.f doesn't get rechecked.

[case testImportSwap]
from a import foo
def f() -> int:
    return foo()

[file a.py]
from b1 import foo
[file a.py.2]
from b2 import foo

[file b1.py]
def foo() -> int:
    return 12
[file b2.py]
def foo() -> str:
    return 'uhoh'
[out]
==
main:3: error: Incompatible return value type (got "str", expected "int")

Same thing if we switch from a importing foo to just defining it.

[case testImportSwap2]
from b import foo
def f() -> int:
    return foo()

[file a.py]
from b import foo
[file a.py.2]
def foo() -> str:
    return 'uhoh'

[file b.py]
def foo() -> int:
    return 12
[out]
==
main:3: error: Incompatible return value type (got "str", expected "int")

Same problem without using from in m.

[case testImportSwap3]
import a
def f() -> int:
    return a.foo()
[file a.py]
from b1 import foo
[file a.py.2]
from b2 import foo

[file b1.py]
def foo() -> int:
    return 12
[file b2.py]
def foo() -> str:
    return 'uhoh'
[out]
==
main:3: error: Incompatible return value type (got "str", expected "int")