feat: add the types package of a mapped dependency to the dev dependencies

[dsherret]

Closes #297
Closes #343

When a remote module is mapped to an npm package, the declaration files that the cdn specifies for it were dropped, so the output failed to type check:

import { parseSVG } from "https://esm.sh/svg-path-parser@1.1.0";

error TS7016: Could not find a declaration file for module 'svg-path-parser'.
  Try `npm i --save-dev @types/svg-path-parser` if it exists...

esm.sh points at the declarations with an X-TypeScript-Types header (https://esm.sh/@types/svg-path-parser@~1.1.6/index.d.ts), so now the package providing them is added to the dev dependencies:

{
  "dependencies": {
    "svg-path-parser": "1.1.0"
  },
  "devDependencies": {
    "@types/svg-path-parser": "~1.1.6"
  }
}

The same applies to a declaration file specified with @deno-types, and the file is no longer emitted into the output when a package provides it.

Details:

• A mapped module isn't in the module graph (a dummy is substituted so it isn't downloaded), so its types header is resolved by fetching it separately. Only modules that the esm.sh and skypack mappers mapped are fetched, so a module the user mapped themselves is still never downloaded. A fetch failure warns and continues rather than failing the build.
• A declaration file that a module imports directly stays in the output.
• The readme documents moving the types package to the dependencies when it shows up in your package's public api.

Verified against the reproductions in both issues: https://esm.sh/svg-path-parser@1.1.0 now type checks, and the react example in #343 emits setIsEnabled: React.Dispatch<React.SetStateAction<boolean>> instead of any.