Partially addresses https://github.com/denoland/deno/issues/25648.
This allows packages that use `crossws` to be installed with `deno
install`. `crossws` specifies an optional peer dependency on
`uWebSockets`, but `uWebSockets` is not on npm (it is used with `git:`
or `github:` specifiers). Previously we would error on this, now we
don't error on non-existent optional peer dependencies.
