Return providers instead of struct from rule impl functions

[alex1545]

This addresses the incompatible change via the ----incompatible_disallow_struct_provider_syntax flag described here.

[alex1545]

This is blocked by file_test returning a struct and scala_import returning a struct.

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[alex1545]

As discussed offline with @nlopezgi, we will merge this now, but not test with the --incompatible_disallow_struct_provider_syntax flag because of external dependencies that still haven't addressed this issue.

[nlopezgi]

lgtm/ Please leave upgrade instructions (and pointer to bazelbuild/bazel-toolchains#383 as example) in commit message so users upgrading can get guidance.

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This pull-request has been approved by: alex1545, nlopezgi
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[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[alex1545]

IMPORTANT
This PR changes the return value type from some rule implementation functions from struct to a list of providers.

No action is required if you are already using the providers returned by the below mentioned rules as described here (e.g. target[ProviderName].fieldName).

Your action may be required if you are using either the implementation function or any targets of the following rules (in your own rule implementation functions or macros):

• container_bundle (in container/bundle.bzl)
• container_image (in container/imagebzl)
• container_import (in container/import.bzl)
• filter_layer (in lang/image.bzl)

Actions Required by Rule

• container_bundle
  If you use the container_images and/or the stamp field of the returned struct, then get these fields from the returned BundleInfo provider as shown here.
• container_image
  If you use the container_parts field of the returned struct, then get this field from the returned ImageInfo provider as shown here
• container_import
  If you use the container_parts field of the returned struct, then get this field from the returned ImportInfo provider (e.g. target[ImportInfo].container_parts instead of target.container_parts)
• filter_layer
  If you use the py field of the returned struct as target.py, then get this field using the returned PyInfo provider as target[PyInfo].

Note
If you access the returned providers with the following syntax:
target.providers[<index>].<field>
then you most likely need to change this to:
target[<index>].<field>