Reduce the number of special case feature groups

[ddbeck]

Some consumers have to handle special groups of features in the structure of the data. We should reduce or eliminate these special cases.

In general, we have a few broad namespaces in BCD, in which features with __compat data reside. The api namespace is typical of this: api.<featurename> is a parent of __compat data. css is more finely divided, with css.properties, css.selectors, etc.

The trouble is within a namespace, such as css.properties.gap. css.properties.gap is not a parent to __compat data. Instead, it's a surprise namespace for flex_context, grid_context, etc. We should avoid these surprises in the structure of the data.

The first step to resolving this would be to make a list of all paths through the structure which are not parents to __compat.

[ddbeck]

This is dupe of #13804 (well, actually, a subset of). Closing in favor of the other issue, since it's a bit more comprehensive.