Add fastify as peer dependency

[mekwall]

This PR adds fastify as a peer dependency so that it works properly with Yarn v2 without having to override the package. Without it you'll get the following message in your log for every plugin since it is trying to require fastify/package.json:

fastify not found, proceeding anyway

Yarn v2 enforces packages to list all dependencies that are used by that package. In this case, fastify is provided by the user so it should be a peer dependency.

Checklist

• [ ] run npm run test and npm run benchmark
• [ ] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [x] commit message and code follows Code of conduct

[jsumners]

I don't think this is correct. It is actually a mistake to use fastify-plugin without a parent fastify instance, i.e. the tree should always look like app => fastify => plugin => fastify-plugin.

This issue also exhibits itself when using pnpm to install modules. In this case, the warning is nothing more than an annoyance.

The solution we decided upon, and no one has had time to implement yet, was discussed in https://github.com/fastify/fastify/issues/1780#issuecomment-569044068

[mcollina]

const fastify = require(require.resolve('fastify', {paths: [require.main.filename]}))

[mcollina]

Fixed in https://github.com/fastify/fastify-plugin/pull/94