Go: rework mock generation

[kormat]

The use of gomocks in our codebase currently has the following issues:

• Generating mocks is not hermetic - it uses whatever version of mockgen is in the user's path.
• There is no CI check to ensure that the generated mocks are consistent with the source.
• There is no way to automatically catch generated mock files that remain after their source has been removed.
• There is no documentation on what setup is required before tools/gomocks will actually work.

Proposed fix:

• Use bazel to build the mockgen binary for the version of github.com/golang/mock we're using, and then use that in tools/gomocks.
• Move from scion.sh mocks to make mocks. That way it can depend on setting up the required env (including compiling the mockgen binary).
• CI needs to regenerate mocks and error out if they differ from the ones in git.
• CI needs to error out if there are mock files with no source.
  • This can be done by either:
  • have a whitelist of mock_X dirs that should exist, and throw an error if any other mock_X dirs exist.
  • nuke all mock_X dirs before regenerating mocks.

[kormat]

Building mockgen with bazel fails because our version of rules_go pulls in a broken version of org_golang_x_tools (it predates https://github.com/golang/tools/commit/202502a5a9245830b5ec7b877e26b67760da8e67). The minimum version of rules_go to fix this seems to be 0.19.0.

[kormat]

Building mockgen with bazel fails with the updated rules_go because... it pulls in a newer version of org_golang_x_tools which has the same problem as the old version, just in a different file: https://github.com/golang/tools/commit/eec4c98bf5a6a9765c9de84cf61e447130fe8e54#diff-1910375731a87c0dea421568450908cc is the fix.