Compat submodules always give "Elaboratable created but never used" traceback

[tdaede]

Testcase:

#!/usr/bin/env python3
from nmigen import *
from nmigen.compat import *

class Compat2(Module):
    pass

class Compat(Module):
    def __init__(self):
        self.submodules += Compat2()

if __name__ == "__main__":
    m = Compat()
    f = Fragment.get(m, platform=None)

Output:

./testcase.py:10: DeprecationWarning: instead of `self.submodules +=`, use `m.submodules +=`
  self.submodules += Compat2()
Elaboratable created but never used
Traceback (most recent call last):
  File "./testcase.py", line 13, in <module>
    m = Compat()
  File "./testcase.py", line 10, in __init__
    self.submodules += Compat2()

[whitequark]

Fixed in 537d918.