This patch introduces the basic infrastructure of the code generation
suite and the `dbscheme` generator.
Notice that the checked in `schema.yml` should reflect swift 5.6 but
might need some tweaking.
Closes https://github.com/github/codeql-c-team/issues/979
* fixed 5.0.0 as bazel version
* made dependencies better loadable
* moved `//swift/install` to `//swift:create-extractor-pack` (following
the clearer ruby naming)
* renamed `extractor_pack` to `extractor-pack` for consistency with Ruby
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.
