Migrate bazel-java-sdk to a dedicated GitHub repo

[plaird]

The intent for the bazel-java-sdk is for it to graduate out of the BEF repo and into it's own GitHub repo. The Java SDK has been written such that it isn't specific to Eclipse, and could be useful for other Bazel tooling projects.

This isolation work has been completed internally to the BEF repo. The top-level bzl-java-sdk directory is the result of this.

But while splitting bzl-java-sdk into a new GitHub repo is the intent, we are doing heavy development on it in conjunction with BEF. So I am resisting that move for now. With Git submodules, we could link them, but that just feels like a pain at this stage of development. I am thinking the move out of bzl-java-sdk will occur at the end of the 1.x release line.

[plaird]

My current thinking on this is to use copybara between BEF and the bazel-java-sdk. I expect BEF work will continue to need to make sizable changes to the SDK, so having it vendored feels like the better approach.

[plaird]

The sdk repo is still private, but I will open up soon. The copybara scripts in the toplevel of this repo copy the code back and forth. I didn't actually use copybara, but I gave them that name because it makes it more obvious what they do.

https://github.com/salesforce/bazel-java-sdk