chore: run lint in a separate build before running the tests

[mdelapenya]

What does this PR do?

It extracts the golangci-lint step to a separate job, so that it can be called separately before the tests.

To detect the changes, we have created a shell script, scripts/changed-modules.sh. This script receives one single env var as input parameter, ALL_CHANGED_FILES, which on CI will be provided by a Github action, https://github.com/tj-actions/changed-files, that puts in there a list of the modified files, comparing the current PR changeset with the parent branch (main). We can tune this up to always check the latest main branch (open to discussion here).

The script will compare all existing modules (looking up all the go.mod files of interest, no test files) with the modified files, building an array of modified modules. At the moment there is modified file in the core, in other words the entire root directory without modulegen/modules/examples, all the modules will be included in the build. There are some exclusions for the code: files in the .devcontainer, .vscode, docs, scripts will not cause all modules will be included. So it could be the case that no modules could end up in the build if only files in those exclusions are changed. In that case, we are going to conditionally skip the lint, test and upload-to-sonar jobs.

Finally, for running the tests, we are going to keep the matrix for running them in multiple Go versions and OSs, with the following rules:

• any Go module will run the tests for the two most recent Go releases (core, modulegen, modules and examples).
• the core module, modules and examples will run the tests on ubuntu-latest.
• the modulegen module will run the tests on ubuntu-latest, macos-latest and windows-latest.
• the jobs for reaper-off and rootless-mode will run the tests for the two most recent Go releases, on `ubuntu-latest, only.

BTW I was able to iterate faster on this at the moment I discovered the power of https://github.com/nektos/act. Fantastic tool!

Why is it important?

There are two major goals:

• Do not run the lint until it's too late in the build.
• Reduce the feedback loop when contributing to modules. At the moment, a module contributor has to wait for all the core changes to be built and pass the tests. With this approach, if the PR touches just one module, then that module will be added to the workflow.

As a drawback, in my opinion, is that two different builds with two different changesets won't run the same set of tests, so they are not deterministic. I think this is a minor detail that would be satisfied with the improved experience for module contributors at CI time. For core maintainers, we will probably run all the modules per PR, but that could be expected to verify we are not breaking the downstream modules.

How to test this PR

The shell script is the key part, and it's explained in the comments for the script. It requires running the script with the required environment variables for the different use cases:

1. A Go file from the core module is modified:
   ALL_CHANGED_FILES="examples/nginx/go.mod examples/foo/a.txt a/b/c/d/a.go" ./scripts/changed-modules.sh
   The output should be: all modules.

2. A file from a module in the modules dir is modified:
   ALL_CHANGED_FILES="modules/nginx/go.mod" ./scripts/changed-modules.sh
   The output should be: just the modules/nginx module.

3. A file from a module in the examples dir is modified:
   ALL_CHANGED_FILES="examples/nginx/go.mod" ./scripts/changed-modules.sh
   The output should be: just the examples/nginx module.

4. A Go file from the modulegen dir is modified:
   ALL_CHANGED_FILES="modulegen/a.go" ./scripts/changed-modules.sh
   The output should be: just the modulegen module.

5. A non-Go file from the core dir is modified:
   ALL_CHANGED_FILES="README.md" ./scripts/changed-modules.sh
   The output should be: all modules.

6. A file from two modules in the modules dir are modified:
   ALL_CHANGED_FILES="modules/nginx/go.mod modules/localstack/go.mod" ./scripts/changed-modules.sh
   The output should be: the modules/nginx and modules/localstack modules.

7. Files from the excluded dirs are modified:
   ALL_CHANGED_FILES="docs/a.md .vscode/a.json .devcontainer/a.json scripts/a.sh" ./scripts/changed-modules.sh
   The output should be: no modules.

Follow-ups

IMPORTANT!: We need to update the branch settings so that the new checks are used to allow the merge.

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