project_legacy_paths menu items break all kinds of things

If you enable project_legacy_paths, all sorts of things become broken:

For example:
project/autocomplete [works]
project/autocomplete/p [page not found]

Another example:
project/issues/update_project

I thought at first this was caused by views that live at project/% taking precedence, but that didn't solve it. Turning off project_legacy_paths did. So, we need to debug this and figure out what's going wrong in there causing it to hijack other paths it shouldn't be caring about.

Futhermore, we might need to change project_legacy_paths a lot more if we change the URLs more, e.g. #180316: fix project/mymodule URI insanity...