Swift: make C++ code generation more self-contained

This is solving a papercut, where the C++ build was relying on the
local dbscheme file to be up-to-date, even if all the information for
building is actually in `schema.yml`. This made a pure C++ development
cycle with changes to `schema.yml` clumsy, as it required a further
dbscheme generation step.

Now for C++ the dbscheme is generated internally in the build files, and
thus a change in `schema.yml` is reflected immediately in the C++ build.

A `swift/codegen` step for checked in generated code (including the
dbscheme) is still required, but a developer can do it just before
running QL tests or committing, instead of during each C++
recompilation.

Some directory reorganization was also carried out, moving specific
generator modules to a new `generators` python package, and only leaving
the two drivers at the top level.

.github/workflows/swift-codegen.yml

@@ -25,8 +25,7 @@ jobs:
           git diff --exit-code --stat HEAD
       - name: Generate C++ files
         run: |
-          bazel run //swift/codegen:trapgen -- --cpp-output=$PWD/swift-generated-headers
-          bazel run //swift/codegen:cppgen -- --cpp-output=$PWD/swift-generated-headers
+          bazel run //swift/codegen:cppcodegen -- --cpp-output=$PWD/swift-generated-headers
       - uses: actions/upload-artifact@v3
         with:
           name: swift-generated-headers