Also removed the version check step, as a version not existing will give
a 404 any way later on, and that was adding a delay.
The cache is stored in a `.kotlinc_zips` and will be cleaned up by
`--clear`.
Those have shown to cause problems with too many concurrent downloads.
This changes kotlinc dependencies fetching to:
* use `resource/kotlinc-dependencies` if available (which is the case
for the internal repo)
* otherwise, download them from maven.
This means sha256 hashes need to be written down for bazel.
