Crash with star imports and new file in the build

[ilevkivskyi]

This test case:

[case testAddedSubStar]
# cmd: mypy -m a pack pack.mod b
# cmd2: mypy -m other
[file a.py]
from pack import *
[file pack/__init__.py]
[file pack/mod.py]
[file b.py]
import pack.mod
[file other.py]
import a
[out]
[out2]

crashes with

/Users/ilevkivskyi/src/mypy/mypy/build.py:157: in build
    result = _build(sources, options, alt_lib_path, flush_errors, fscache)
/Users/ilevkivskyi/src/mypy/mypy/build.py:207: in _build
    graph = dispatch(sources, manager)
/Users/ilevkivskyi/src/mypy/mypy/build.py:2214: in dispatch
    process_graph(graph, manager)
/Users/ilevkivskyi/src/mypy/mypy/build.py:2505: in process_graph
    process_fresh_modules(graph, prev_scc, manager)
/Users/ilevkivskyi/src/mypy/mypy/build.py:2591: in process_fresh_modules
    graph[id].fix_cross_refs()
/Users/ilevkivskyi/src/mypy/mypy/build.py:1598: in fix_cross_refs
    self.options.use_fine_grained_cache)
/Users/ilevkivskyi/src/mypy/mypy/fixup.py:25: in fixup_module
    node_fixer.visit_symbol_table(tree.names)
/Users/ilevkivskyi/src/mypy/mypy/fixup.py:77: in visit_symbol_table
    self.quick_and_dirty)
/Users/ilevkivskyi/src/mypy/mypy/fixup.py:261: in lookup_qualified_stnode
    return lookup_fully_qualified(name, modules, raise_on_missing=not quick_and_dirty)
/Users/ilevkivskyi/src/mypy/mypy/lookup.py:47: in lookup_fully_qualified
    assert key in names, "Cannot find %s for %s" % (key, name)
AssertionError: Cannot find mod for pack.mod

I think this is high priority because the crash affects users who adds new files to typed builds in existing code bases. There must be another way to reproduce this where all files from first command are also present in the second, but I didn't find a simple version of it.

Anyway, the crux seems to be the interference between lack of dependencies from star import (that can also import modules), and add_submodules_to_parent_modules().