Make namespace and namespace-alias searches (and their ref searches) work. Ref bug 1082100.
Delete a compiled artifact that somehow got committed.
Give group_by_line a better name.

We're padding out a bunch of span-labeled needles into a series of one iterable per line, not bucketing them.
Make warning and warning-opt searches work. Ref bug 1030312.

* Refactor to create _NameFilter, a single-value filter for needles without qualnames. _SymbolFilter becomes _QualifiedNameFilter. Some symbols, like macros, don't have qualnames [that are meaningfully different from regular names], after all.
* Rename symbol_needles() to qualified_needles() for consistency.
* Make WarningFilter and WarningOptFilter _NameFilters, along with MacroFilter and MacroRefFilter so I don't forget later.
* Remove a unit test that is now invalid and is redundant with integration tests.
* Unskip warnings tests. Amend expected strings. We no longer (at least for now) support wildcard searches in every little filter. That's part of why SQLite-based DXR was so slow.
Kill an unused import.
Delete some of the old, now-unused needle machinery.
Tighten up the environment() specs.