refactor build plan pipeline

Previously the BuildPlan pipeline didn't execute generators and
transformers concurrently.  All steps were sequentially executed.  Holos
was primarily concurrent by executing multiple BuildPlans at once.

This patch changes the Build implementation for each BuildPlan to
execute a GoRoutine pipeline.  One producer fans out to a group of
routines each executing the pipeline for one artifact in the build plan.
The pipeline has 3 stages:

1: Fan-out to build each Generator concurrently.
2: Fan-in to build each Transformer sequentially.
3: Fan-out again to run each validator concurrently.

When the artifact pipelines return, the producer closes the tasks
channel causing the worker tasks to return.

Note the overall runtime for 8 BuildPlans is roughly equivalent to
previously at 160ms with --concurrency=8 on my M3 Max.  I expect this to
perform better than previously when multiple artifacts are rendered for
each BuildPlan.

cmd/holos/tests/v1alpha5/guides/helm.txt

@@ -7,11 +7,11 @@ cd $WORK
 exec holos generate platform v1alpha5 --force
 
 # Platforms are empty by default.
-exec holos render platform ./platform
+exec holos render platform
 stderr -count=1 '^rendered platform'
 
 # Holos uses CUE to build a platform specification.
-exec cue export --expression holos --out=yaml ./platform
+exec holos show platform
 cmp stdout want/1.platform_spec.yaml
 
 # Define the host and port in projects/blackbox.schema.cue
@@ -22,7 +22,7 @@ mv projects/platform/components/prometheus/prometheus.cue.disabled projects/plat
 mv platform/prometheus.cue.disabled platform/prometheus.cue
 
 # Render the platform to render the prometheus chart.
-exec holos render platform ./platform
+exec holos render platform
 stderr -count=1 '^rendered prometheus'
 stderr -count=1 '^rendered platform'
 cmp deploy/components/prometheus/prometheus.gen.yaml want/1.prometheus.gen.yaml
@@ -73,8 +73,8 @@ core.#BuildPlan & {
 	metadata: name: _Tags.component.name
 }
 -- want/1.platform_spec.yaml --
-kind: Platform
 apiVersion: v1alpha5
+kind: Platform
 metadata:
   name: default
 spec:

internal/artifact/artifact.go

@@ -20,6 +20,7 @@ var _ Store = NewStore()
 type Store interface {
 	Get(path string) (data []byte, ok bool)
 	Set(path string, data []byte) error
+	// Save previously set path to dir preserving directories.
 	Save(dir, path string) error
 }