Adds a new:
- gazelle generator
- rule + rule targets + catchall target
for generating go-mockgen mocks & testing for their being up-to-date.
Each go_mockgen macro invocation adds targets for generating mocks, copying to the source tree, as well as testing whether the current source tree mocks are up-to-date.
How to use this: `bazel run //dev:go_mockgen` for the catch-all, or `bazel run //some/target:generate_mocks` for an individual package, and `bazel test //some/target:generate_mocks_tests` to test for up-to-date-ness. There is no catch-all for testing
This currently uses a fork of go-mockgen, with an open PR for upstream here: https://github.com/derision-test/go-mockgen/pull/50.
Closes https://github.com/sourcegraph/sourcegraph/issues/60099
## Test plan
Extensive testing during development, including the following cases:
- Deleting a generated file and its entry in a go_library/go_test `srcs` attribute list and then re-running `sg bazel configure`
- Adding a non-existent output directory to mockgen.test.yaml and running the bash one-liner emitted to prepare the workspace for rerunning `sg bazel configure`
The existing config tests a lot of existing paths anyway (creating mocks for a 3rd party library's interface, entries for a given output file in >1 config file etc)
