Various issues with bazel_source

Due to the complexity of external dependencies in bazel, the bazel_source plugin inevitably misses some issues. In building various bazel projects thus far I have found the following issues:

• url and urls are valid to define urls. Currently the plugin skips urls specified with url, which causes build errors. Fixed in !61 (merged)
• Bazel does not require a sha256 of sources. Currently the plugin skips urls without a sha256, which causes build failures. I'd think this should always be a warning, with configurable failure at fetch time personally. Fixed in !72 (merged)
• git_repository is entirely unhandled. At the moment bazel will pull this using git directly at build time, which obviously fails. I've not yet scoured a manifest file to see what it looks like.

I also anticipate more issues involving external dependencies like this, I'm yet to try anything go or maven related for example, and I expect issues with this. I have successfully built a python project, which you can see at https://gitlab.com/celduin/burn-sdk/merge_requests/7 (Depends on !61 (merged)).