make nebari API private

[pmeier]

Implements 4. of nebari-dev/governance#37 in three steps:

1. 62379f4: Introduce a src/nebari layout. Right now we have the package directly in our repository root. Since we will ultimately have a nebari as well as a _nebari package, it is useful to tuck them together in one folder. This is done by pytest, which we are following with this API change, and many other Python projects as well.
2. be85b66: make nebari package private by renaming it to _nebari.
3. 3181f57: reintroduce a public nebari package that does nothing but expose the CLI and a __version__ attribute.

I'll send one commit per step outlined above to make it easier to review. Since the diff will get quite large, I advise reviewers to go commit by commit.

This PR will conflict with basically every other open PR and every patch that was merged into develop after I have started. To keep disruption to a minimum, I plan to only rebase once after this PR has been approved. For all other open PRs, I'm available for helping to migrate them. Tag me there if you need help.

[github-actions]

💰 Infracost estimate: monthly cost will not change

Project	Previous	New	Diff
All projects	$73	$73	$0

8 projects have no cost estimate changes.

Infracost output

──────────────────────────────────
The following projects have no cost estimate changes: nebari-dev/nebari/data/stages/01-terraform-state/gcp (Module path: 01-terraform-state/gcp), nebari-dev/nebari/data/stages/02-infrastructure/gcp (Module path: 02-infrastructure/gcp), nebari-dev/nebari/data/stages/03-kubernetes-initialize (Module path: 03-kubernetes-initialize), nebari-dev/nebari/data/stages/04-kubernetes-ingress (Module path: 04-kubernetes-ingress), nebari-dev/nebari/data/stages/05-kubernetes-keycloak (Module path: 05-kubernetes-keycloak), nebari-dev/nebari/data/stages/06-kubernetes-keycloak-configuration (Module path: 06-kubernetes-keycloak-configuration), nebari-dev/nebari/data/stages/07-kubernetes-services (Module path: 07-kubernetes-services), nebari-dev/nebari/data/stages/08-nebari-tf-extensions (Module path: 08-nebari-tf-extensions)
Run the following command to see their breakdown: infracost breakdown --path=/path/to/code

──────────────────────────────────
11 cloud resources were detected:
∙ 4 were estimated, all of which include usage-based costs, see https://infracost.io/usage-file
∙ 7 were free:
  ∙ 6 x google_project_iam_member
  ∙ 1 x google_service_account

This comment will be updated when the cost estimate changes.

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[pmeier]

All CI failures on 62379f4 are tracked in #1817. As discussed in yesterday's community meeting, I'm going to ignore these and move forward with the PR.

[pmeier]

Same deal as in #1778 (comment) with be85b66 and 3181f57.

[costrouc]

Looks great to me @pmeier and I'm in favor of merging this earlier rather than later. Once you give the merge conflicts I'm in favor of immediately merging.

I like our minimal nebari interface now of nebari/__init__.py and nebari/__main__.py