[catalog-server] Bootstrapping packages

We need to have the catalog do an initial search / crawl of packages with custom elements manifests. We can try to find such packages in a few ways:

1. Include a hard-coded list of packages that we know have custom elements
2. Search npm for keywords like ["web components"](https://www.npmjs.com/search?q=keywords%3A%22web%20components%22), "custom elements", etc via the [search endpoint](https://github.com/npm/registry/blob/master/docs/REGISTRY-API.md#get-v1search).
3. Scrape the list of packages that depend on well-known web component libraries like [Lit](https://www.npmjs.com/browse/depended/lit), Stencil, Polymer, FAST, LWC, etc. There doesn't appear to be a registry API for this.