Bazel build needs adjustments for open-source

[aaronmondal]

Describe the bug
The current Bazel build files are inconsistent and appear to rely on an internal wrapper and internal rulesets which cannot be found in an open-source context. At the moment it is not possible to build the project with Bazel.

To Reproduce
Add WORKSPACE.bazel and BUILD.bazel files with a rules_scala setup and attempt to build the repository:

bazel build ...

Expected behavior
The build to pass.

Environment
I'm testing this from the Nix flake proposed in #441. A regular bazel installation will yield the same results.

Additional context
This actually seems not too hard to fix, but the starlark files will need to be globally refactored before a fix becomes viable in terms of commit reviewability. I think something like this could work:

1. • Unify all build files with buildifier.
2. • Use buildozer to globally add load statements where needed.
3. • Add name attributes where needed.
4. • Change absolute paths to proper labels.
5. • Migrate custom rules (hadoop_binary, jvm_binary etc) to open-source equivalents or upstream them to open-source repos or provide their implementation as part of this repository.

Much of this can be automated with buildozer. For instance, to add loads:

buildozer 'new_load @io_bazel_rules_scala//scala:scala.bzl scala_library' //...:*
buildozer 'fix unusedLoads'