Support deno add with aliases

[dsherret]

For example, npm is:

npm install --save-dev typescript-3.6.3@npm:typescript@3.6.3 typescript-4.0.2@npm:typescript@4.0.2

We should probably have the exact same syntax:

deno add typescript-3.6.3@npm:typescript@3.6.3 typescript-4.0.2@npm:typescript@4.0.2

Or with https urls:

deno add path@https://deno.land/std/path/mod.ts fs@https://deno.land/std/fs/mod.ts