This commits a bazel-based build system for C#
using `rules_dotnet`. External dependencies are managed
via `paket`, and updates to the generated bazel files
are done via `./update-deps.sh`.
We're providing our own (minimal) test runner for `xunit`
tests.
It turns out everything that is needed for the installer to work on
windows is enabling runfiles. This also requires symlinks to avoid
excessive copying of files.
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.
