As a prerequisite to start injecting BDD mocks for our K8s backend
dependencies, this commit restructures the Catalyst code to use
singletons in a hierarchical package structure.

The singletons are necessary in order to be able to inject mocks
for BDD tests. The reasons for the hierarchical packages are a bit
more complicated, but they have to do with the fact that the new
`setup.go` file will be used by the BDD test runner.

`setup.go` cannot be in the main package or else would not be
accessible by the runner. But if `setup.go` is not in main, then it
can't access the rest of the code that lived in main (`router.go`,
`catalyst.go`, etc). All that other code then needs to be moved to
its own packages as well.

Finally moving important pieces of the code out of main is a good
improvement on its own though, since now any cycle dependencies
between packages get exposed (cycle package deps being something
normal in other languages but something Go frowns upon)

Bug: T367782