Always generate secondary instance for selects

[lognaturel]

Software and hardware versions

pyxform v1.9.0

We've had various conversations about selects and when they're generated as inline/static or as secondary instances/dynamic starting with #203. There are various issues around them such as #387 (comment) and #590

In #590 we are coming to the conclusion that it'd be better to always generate secondary instances and use itemsets.

I believe there are currently two cases that require inline/static choices:

• or_other -- what's the current behavior with a localized form? Maybe we could inject a choice into a secondary instance instead? The ultimate goal is to deprecate this functionality: Add support for translating or_other? #218 (comment) so I think we should preserve it in whatever way is easiest.
• adding extra choices to a select with the search() appearance. I don't remember the details of how this works but I think Collect might need static choices for that (I will check and update)

Any other cases that might be special or need static selects?

[lognaturel]

@lindsay-stevens You up for taking this on soon? Let me know if you want to discuss before diving in.

[lindsay-stevens]

This is in progress. It'll be a big PR because the initial change (i.e. ignoring cases requiring static for now) causes ~30 tests to fail. For some of the tests I'm doing a quick-fix on, such as those testing internals e.g. builder_tests.py. Others using assertPyxformXform are being refactored from xml__contains to xml__xpath_match as part of fixing them, which sometimes takes a bit of git archaeology if the test intent isn't obvious.