ModuleLoader: Avoid deadlock by retrying operation dispatch

[radeksimko]

This fixes a bug which is described in the inline comment. It seems quite unlikely end-user would hit it, but I did manage to hit it in CI test (TestModuleManager_ModuleCandidatesByPath) which panicked due to timeout (which I unfortunately do not have available as I retried it and it passed).

I also verified at least once that with sufficient pressure the codepath is hit:

$ go test ./internal/terraform/module -run=TestModuleManager_ModuleCandidatesByPath -count=750 -timeout=5s -p=1 -v | grep "no available capacity"
2021/02/03 15:29:59 module_loader.go:83: no available capacity, retrying dispatch

but I couldn't get a reliable repro unfortunately.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the context necessary to investigate further.