Swift: first skeleton extractor

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.

defs.bzl

@@ -0,0 +1,19 @@
+codeql_platform = select({
+    "@platforms//os:linux": "linux64",
+    "@platforms//os:macos": "osx64",
+    "@platforms//os:windows": "win64",
+})
+
+_paths_bzl = """
+def source_dir():
+    return '%s/' + native.package_name()
+"""
+
+def _ql_utils_impl(repository_ctx):
+    root = repository_ctx.path(Label("@ql//:WORKSPACE.bazel")).realpath.dirname
+    repository_ctx.file("BUILD.bazel")
+    repository_ctx.file("paths.bzl", content = _paths_bzl % root)
+
+ql_utils = repository_rule(
+    implementation = _ql_utils_impl,
+)