"<function> defined here" notes omitted when function is loaded from cache

[msullivan]

mypy emits a nice " defined here" note when calling a function with unexpected keywords arguments.

Unfortunately the note is omitted if the module the function is defined in has been loaded from the cache.
This both eliminates a nice message (which we should add more of!) and causes annoying deviations between modes.

The following coarse-grained incremental test fails

[case testCachedUnexpectedKeywordArgument]
import a
[file a.py]
import b
b.lol(uhhhh=12) # tweak
[file a.py.2]
import b
b.lol(uhhhh=12)
[file b.py]
def lol() -> None: pass
[out]
tmp/a.py:2: error: Unexpected keyword argument "uhhhh" for "lol"
tmp/b.py:1: note: "lol" defined here
[out2]
tmp/a.py:2: error: Unexpected keyword argument "uhhhh" for "lol"
tmp/b.py:1: note: "lol" defined here

As well as this fine-grained incremental test:

[case testUnexpectedKeywordArgument]
import a
[file a.py]
import b
[file a.py.2]
import b
b.lol(uhhhh=12)
[file b.py]
def lol() -> None: pass
[out]
==
a.py:2: error: Unexpected keyword argument "uhhhh" for "lol"
b.py:1: note: "lol" defined here

[emmatyping]

It seems this may be blocking #1412.

[ilevkivskyi]

I tried to fix this by serializing just the necessary parts for these better error messages (serializing full definition would be too expensive), but here is a problem. If I write the original definition line number to the cache, then it becomes part of public interface (what is called interface hash in build.py). This is problematic, since it would trigger to many other files to recheck. This only affects coarse grained runs, but in any case it seems wrong to make line number a part of public interface. So if we want better error messages, we need to rethink how we determine public interface, so that we can store some extra metadata.