Fix source for drill queries

[christopherswenson]

Currently, drill queries only work when the source is a named explore that appears in the model. Queries with inline tables, inline explores, from(...), or queries with multiple pipeline stages all do not work.

To fix this, we need more metadata to appear within StructDefs about their source.

@mtoy-googly-moogly @lloydtabb

[mtoy-googly-moogly]

Drilling lives in that uncomfortable where it is really only understood at the level of textual substitution, which is why drilling is broken if we can't write a new model as a drilled version of an old model.

If we understood drilling more deeply, probably a query would know how to drill itself, and it would have methods you could call to ask it to do that.

[christopherswenson]

Yeah, I see that as the correct direction for drilling

[anikaks]

Had someone stumble on this today; I'm wondering whether we want to hide the drill copy shortcut when the drilling won't work for now if we don't think this is likely to happen in the near term