Blueprints: Named resource overrides

[adamziel]

Let's introduce runtime-specific "resource overrides" to make Blueprint development easier. For example:

• In Playground CLI, we could say --resource-override=gutenberg:./gutenberg/dist to load a local Gutenberg build instead of a fetching a remote zip file.
• In Playground webapp, we could say ?resource-override=gutenberg:https://mysite.com/gutenberg.zip to load Gutenberg build from a remote zip file. We could also say ?resource-override=gutenberg:local-dir to trigger a directory picker and load a local Gutenberg build.

This would be highly useful in development and as a Blueprints builder feature.

Motivation

Building a Blueprint using external resources may require a couple of iterations on those resources. The typical process I've seen is this:

1. Bundle the dependencies
2. Push them to GitHub
3. Wait a moment for the github.com cache to expire
4. Update the Blueprint
5. Reload Playground with the updated Blueprint
6. Spot another issue, adjust the dependency, start over

This is annoying and time consuming. Playground would be so much more useful if it could just load a bunch of local files.

Implementation

Blueprints could support a new top-level "resources" key that would list all the overridable resources:

{
	"resources": {
		"gutenberg": {
			"resource": "wp.org/plugins",
			"slug": "gutenberg"
		}
	},
	"steps": [
		{
			"step": "installPlugin",
			"pluginData": { "ref": "gutenberg" }
		}
	]
}

From there, runtimes would be free to accept a list of label:URI overrides to adjust these top-level resources before resolving them.

This is also a step towards the Blueprints v2 schema.