Fix some incompatibilities in Bazel 0.22.0

[jayconrod]

• go_proto_library now relies on the undocumented ProtoInfo instead of
  the .proto field.
• go_proto_library now uses to_list() when iterating ProtoInfo
  depsets.
• A test in tests/core/cgo now uses cc_binary to generate shared
  libraries.

Updates #1888

[jayconrod]

TODO:

• Migrate cgo to CcInfo per instructions at incompatible_disable_legacy_cc_provider: Deprecate old C++ Starlark provider API bazel#7036.
• Verify this works and other things aren't broken on macOS and Windows.
• This probably requires bumping the minimum version of Bazel.

[jayconrod]

We can't start using CcInfo and ProtoInfo until our minimum version is Bazel 0.22.0 (which is not released yet). Those symbols are resolved eagerly by the Starlark interpreter, so the only way not to use them on older versions is to load .bzl files through a repository rule that dynamically selects which .bzl file to load. rules_go currently loads everything from def.bzl, so this would require splitting workspace definitions and rule definitions into two files, which would be a breaking change for everyone. I think it's better to just keep release branches that work with old / new versions of Bazel.

This PR will be cherry-picked onto older branches, which need to continue supporting older versions of Bazel. Another PR will migrate to CcInfo and ProtoInfo, probably in rules_go 0.18.0 (not the coming release but the one after).