(pipelines): FileSets rendering of `cdk.out` artifacts conflicts with artifacts codebuild partial

### Describe the bug

The `pipelines` module in the `CodePipeline` construct, when setup to be self-mutating, takes any `primary_output_directory` property defined for the synth `ShellStep` as well as any additional output directories added, and renders into a codebuild `artifacts:` partial section. If the construct user adds an `artifacts:` partial to codebuild options to the `CodePipeline` constructor (`code_build_defaults` or `synth_code_build_defaults`) then it is rendered with 2 `artifacts:` sections in the buildspec defined for the `CodeBuild` resource. This results in a jsii synth error of `jsii.errors.JSIIError: Error: Only one build spec is allowed to specify artifacts.`

Example result
```
artifacts:
secondary-artifacts:
dist:
base-directory: dist
files:
- "**/*"
artifacts:
base-directory: cdk.out
files:
**/*
```

### Expected Behavior

I would expect some sort of hash merge to occur between FileSets defined for the ShellStep (whether default added `cdk.out` directory or something custom from the library user) as well as any `artifacts:` section in any codebuild partials in play.

### Current Behavior

Currently two `artifacts:` hashes exist in the same rendered codebuild partial and jsii is choking on it.

### Reproduction Steps

```
synth_code_build_defaults = pipelines.CodeBuildOptions(
partial_build_spec=codebuild.BuildSpec.from_object({"artifacts":{
"secondary-artifacts":{"dist":{"base-directory":"dist","files":["**/*"]} } } })
)
synth_shell_step = pipelines.ShellStep(
"Synth",
input=pipelines.CodePipelineSource.connection(redacted),
install_commands=["./install.sh"],
commands=["./build.sh"]
)
pipeline=pipelines.CodePipeline(
self,
"Pipeline",
pipeline_name="test-Pipeline",
synth=synth_shell_step,
synth_code_build_defaults=synth_code_build_defaults,
)
pipeline.build_pipeline()
```

and synth should fail with `jsii.errors.JSIIError: Error: Only one build spec is allowed to specify artifacts.`

### Possible Solution

I tried to dig into how `pipelines/codepipeline.ts` is handling artifacts (and how that relates to `aws-codepipeline/pipeline.ts` and really wasn't able to.

### Additional Information/Context

_No response_

### CDK CLI Version

2.43.0 (build 487870a)

### Framework Version

_No response_

### Node.js Version

v14.17.0

### OS

Linux

### Language

Python

### Language Version

Python 3.8.5

### Other information

_No response_

Start by reading pipelines/codepipeline.ts and then the related aws-codepipeline/pipeline.ts handling of CodeBuild partials and FileSets. Reproduce the Python example with synth_code_build_defaults and a ShellStep, then verify that the rendered buildspec contains one merged artifacts section and no jsii error.

Written by the indexing model from the issue text.