pkg/gcp/gcs: Improve GCS path construction/retrieval logic

[justaugustus]

What type of PR is this?

/kind bug cleanup

What this PR does / why we need it:

Continuation of #1711.

• pkg/gcp/gcs: Improve path normalization logic

  • Ensure constructed paths are prefixed with gs://

  • Use flag vars instead of strings for RsyncRecursive

  • Add isPathNormalized function

    Use this function as pre-check for any gsutil/GCS functions that
    manipulate GCS bucket contents.

  • Use isPathNormalized in RsyncRecursive and PathExists

  • Clarify when we are constructing a release path or version marker path

• pkg/gcp/gcs: NormalizeGCSPath now handles multiple path elements

  This allows any caller to pass an arbitrary GCS path elements without
  having to handle any filepath.Join() logic.

  Should handle the following cases:

  • single path element
  • multiple path elements
  • path element already contains gs://
  • path element was the result of a filepath.Join() and contains gs:/
  • path element contains a leading /

  Should error if:

  • there were no path elements
  • the only path element is an empty string

• pkg: Update usages of gcs.NormalizeGCSPath()

• pkg/release: Respect the GCS suffix when pushing version markers

• pkg/gcp/gcs: Add a few test cases for GCS paths

• pkg/build: Fixup filepath.Join() bug when pushing release artifacts

• pkg/build: Deprecate the GCSSuffix option and introduce GCSRoot option

  Testing with the --gcs-suffix flag has been frustrating.

  It's possible the original shell library never properly handled this
  option and it was never noticed because the option isn't exercised in
  our releases or CI jobs.

  Here we replace its usage with --gcs-root.

  If specified, it will override BuildType.

  When unset:

  • BuildType: "ci"
  • final path: gs:///ci

  When set:

  • BuildType: "ci"
  • GCSRoot: "new-root"
  • final path: gs:///new-root

• pkg/build: Drop references to GCSSuffix

• pkg/gcp/gcs: RsyncRecursive doesn't need to run against normalized paths

  gsutil rsync doesn't actual require that either of the specified
  directories be GCS paths, so here we remove that requirement.

• pkg/release: Fix prefix check to determine bucket permissions

• dependencies.yaml: Add entry for skopeo

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

- pkg/gcp/gcs: Improve path normalization logic
  
  - Ensure constructed paths are prefixed with `gs://`
  - Use flag vars instead of strings for RsyncRecursive
  - Add isPathNormalized function
  
    Use this function as pre-check for any gsutil/GCS functions that
    manipulate GCS bucket contents.
  
  - Use isPathNormalized in RsyncRecursive and PathExists
  - Clarify when we are constructing a release path or version marker path

- pkg/gcp/gcs: NormalizeGCSPath now handles multiple path elements
  
  This allows any caller to pass an arbitrary GCS path elements without
  having to handle any filepath.Join() logic.
  
  Should handle the following cases:
  - single path element
  - multiple path elements
  - path element already contains `gs://`
  - path element was the result of a filepath.Join() and contains `gs:/`
  - path element contains a leading `/`
  
  Should error if:
  - there were no path elements
  - the only path element is an empty string

- pkg: Update usages of gcs.NormalizeGCSPath()

- pkg/release: Respect the GCS suffix when pushing version markers

- pkg/gcp/gcs: Add a few test cases for GCS paths

- pkg/build: Fixup filepath.Join() bug when pushing release artifacts

- pkg/build: Deprecate the GCSSuffix option and introduce GCSRoot option
  
  Testing with the `--gcs-suffix` flag has been frustrating.
  
  It's possible the original shell library never properly handled this
  option and it was never noticed because the option isn't exercised in
  our releases or CI jobs.
  
  Here we replace its usage with `--gcs-root`.
  
  If specified, it will override BuildType.
  
  When unset:
    - BuildType: "ci"
    - final path: gs://<bucket>/ci
  
  When set:
    - BuildType: "ci"
    - GCSRoot: "new-root"
    - final path: gs://<bucket>/new-root

- pkg/build: Drop references to GCSSuffix

- pkg/gcp/gcs: RsyncRecursive doesn't need to run against normalized paths
  
  `gsutil rsync` doesn't actual require that either of the specified
  directories be GCS paths, so here we remove that requirement.

- pkg/release: Fix prefix check to determine bucket permissions

[justaugustus]

/assign @saschagrunert @hasheddan @cpanato
cc: @kubernetes/release-engineering

[saschagrunert]

/lgtm

[k8s-ci-robot]

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