tools: Fix incompatible dependencies for new Gitaly linter

In d1c74493 (Add golangci-lint custom linter infrastructure, 2023-02-15), we have added a Gitaly-specific custom linter that is plugged into golangci-lint. For the time being, we agreed to just make it use the same go.mod as the main module, mostly because we wanted make test to also execute its tests.

This is causing problems already, though. The Renovate bot has decided to immediately try and upgrade the new dependency on golang.org/x/tools to the most recent version. But this causes CI failures now because golangci-lint requires all plugins to always use the same version of that package as it uses itself.

Fix this by moving the Gitaly-specific linting infrastructure into a subpackage of tools/golangci-lint and make it reuse the go.mod file. This ensures that they always use compatible versions and that the main production-level go.mod file is not tied to a specific dependency version.

While at it, fix the missing dependency on its go.sum file.