fix: look for MODULE.bazel in child repos to remove need for stub WORKSPACE

[chickenandpork]

TL;DR

This minimal change in this PR allows me to use example/child workspaces for integration test without creating empty WORKSPACE files.

The Challenge

Following the README.md to implement rules_bazel_integration_test, I found that the child_workspaces needed to have stubbed/empty WORKSPACE files in them; creating empty files caused the error to disappear:

$TEST_TMPDIR defined: output root default is '/tmp/bazel' and max_idle_secs default is '15'.
Computing main repo mapping: 
Loading: 
Loading: 0 packages loaded
Analyzing: 104 targets (12 packages loaded, 0 targets configured)
Analyzing: 104 targets (12 packages loaded, 0 targets configured)

ERROR: /home/runner/work/rules_synology/rules_synology/examples/BUILD.bazel:29:24: in select_workspace_file rule //examples:example-docker-project_bazel_workspace_file: 
Traceback (most recent call last):
	File "/tmp/bazel/_bazel_runner/7eab14037c39b64dde22b5055c5cbc0f/external/rules_bazel_integration_test~/bazel_integration_test/private/bazel_integration_test.bzl", line 24, column 9, in _select_workspace_file_impl
		fail("Can't find WORKSPACE or WORKSPACE.bazel in %s" % ctx.attr.workspace_path)
Error in fail: Can't find WORKSPACE or WORKSPACE.bazel in example-docker-project
ERROR: /home/runner/work/rules_synology/rules_synology/examples/BUILD.bazel:29:24: Analysis of target '//examples:example-docker-project_bazel_workspace_file' failed
ERROR: Analysis of target '//examples:example-docker-project_bazel_workspace_file' failed; build aborted
INFO: Elapsed time: 1.185s, Critical Path: 0.01s
INFO: 13 processes: 13 internal.
ERROR: Build did NOT complete successfully
FAILED: 
Error: Process completed with exit code 1.

This seems to be that the lines modified are not looking for MODULE.bazel files as a signal that a valid child workspace exists:

    workspace_file = paths.join(ctx.attr.workspace_path, "WORKSPACE")
    workspace_bazel = paths.join(ctx.attr.workspace_path, "WORKSPACE.bazel")
    # no `MODULE.bazel` :(
    
    if file.path.endswith(workspace_file) or file.path.endswith(workspace_bazel):
        return [DefaultInfo(files = depset([file]))]
    fail("Can't find WORKSPACE or WORKSPACE.bazel in %s" % ctx.attr.workspace_path)

Suggested Fix

This minimal change in this PR allows me to re-remove the empty WORKSPACE files and have proper functionality back with a bzlmod-formatted ruleset, with my child_workspaces -- my examples -- detected by presence of MODULE.bazel as I would expect. The user can choose existing WORKSPACE, WORKSPACE.bazel, MODULE.bazel, or REPO.bazel as can be seen in the PR.

NOTE 1: I'm unsure whether REPO.bazel is appropriate here.

NOTE 2: if you test this, you may need to use an absolute path on your local_path_override() for the rules_bazel_integration_test : I found that a relative path caused the child workspaces to look in the run files location for a rules_bazel_integration_test whereas I had my working copy parallel/peer to my ruleset that I was developing.

[cgrindel]

Thank you!