Address wagon aliasing issues

As it turns out, not fully testing new features is not necessarily a
good idea ...

This patch follows up 1F616EMO's patch by
* Making get_wagon_prototype return the resolved alias,
* Handling recursive wagon alises (in particular, loops), and
* Adding (partial) unittest for the wagon aliasing system. [v2]: The
testcases are complemented a bit more to cover situations where the
alias resolution system should return nil.

[v2]: This patch should hopefully also warn about not spawning wagons.
Note that this only warns about the missing wagon entity and does _not_
actually fix the issue.

How to test:
* In a world with both advtrains_train_subway and advtrains_train_japan
  enabled, place a subway wagon, a Japanese engine, and a regular
  Japanese wagon.
* Add the test mod to the world; do NOT remove advtrains_train_japan.
* Restart the world. Notice that the Japanese wagons still appear as
  Japanese wagons despite being aliased to subway wagons.
* Restart the world without the advtrains_train_japan mod. Notice that
  the engine appears as the subway wagon while the regular Japanese
  wagon appears as the wagon placeholder. [v2]: Also note that the
  warning message about the missing wagon prototype still mentions the
  regular Japanese wagon.
* Restart the world again with the advtrains_train_japan mod. Notice
  that both type of Japanese wagons reappear as Japanese wagons.
* Observe that unittests work.

Test mod:
advtrains.register_wagon_alias("advtrains:engine_japan", "advtrains:subway_wagon")
advtrains.register_wagon_alias("advtrains:wagon_japan", "advtrains:wagon_japan")

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