1. There was a lot of cloning going on with `NpmPackageInfo`. This is
now stored in an `Arc<NpmPackageInfo>` and cloning only happens on the
individual version.
2. The package cache is now cleared from memory after resolution.
3. This surfaced a bug in `deno cache` and I noticed it can be more
efficient if we have multiple root specifiers if we provide all the
specifiers as roots.
This commit adds support for npm specifier in "deno cache" subcommand.
```
$ deno cache --unstable npm:vite npm:chalk https://deno.land/std/http/file_server.ts
```
Besides downloading requested npm package(s), it will also download
necessary code from "std/node/".