Fix sneaky import cycle

[dairiki]

There is a sneaky import cycle that is exposed by running:

$ python -c "import lektor.types"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/dairiki/git/web/lektor-packages/lektor/lektor/types/__init__.py", line 11, in <module>
    from lektor.types.multi import CheckboxesType
  File "/home/dairiki/git/web/lektor-packages/lektor/lektor/types/multi.py", line 4, in <module>
    from lektor.environment.expressions import Expression
  File "/home/dairiki/git/web/lektor-packages/lektor/lektor/environment/__init__.py", line 28, in <module>
    from lektor.types import builtin_types
ImportError: cannot import name 'builtin_types' from partially initialized module 'lektor.types' (most likely due to a circular import) (/home/dairiki/git/web/lektor-packages/lektor/lektor/types/__init__.py)

This is import-order dependent. If one imports lektor.environment before importing lektor.types all is well.

(It appears this cycle was introduced in 7c68f3f78.)

Description of Changes

• Added a unit test that exercises the bug
• Fixed the problem
• Added unit test(s) covering the changes (if testable)

[yagebu]

Good catch and that test is very good to have :)