Currently, we have a number of assertions in the codebase and certain
assumptions about the AST. These don't always hold, sometimes leading to
a crash in the extractor.
The crashes leave incomplete TRAP files that cannot be imported into the
database.
With this change, we still get those incomplete TRAP files, but we also
get a database in the end (even thoough it is also incomplete as we
cannot import everything).
This adds a first dummy extractor for swift.
Running `bazel run //swift:install` will create an `extractor_pack`
directory in `swift`. From that moment providing `--search-path=swift`
will pick up the extractor.
