add a resolve API for plugins

[evanw]

Plugins now have access to a new API called resolve that runs esbuild's path resolution logic and returns the result to the caller. This lets you write plugins that can reuse esbuild's complex built-in path resolution logic to change the inputs and/or adjust the outputs. Here's an example:

let examplePlugin = {
  name: 'example',
  setup(build) {
    build.onResolve({ filter: /^example$/ }, async () => {
      const result = await build.resolve('./foo', { resolveDir: '/bar' })
      if (result.errors.length > 0) return result
      return { ...result, external: true }
    })
  },
}

This plugin intercepts imports to the path example, tells esbuild to resolve the import ./foo in the directory /bar, and then forces whatever path esbuild returns to be considered external. Here are some additional details:

• If you don't pass the optional resolveDir parameter, esbuild will still run onResolve plugin callbacks but will not attempt any path resolution itself. All of esbuild's path resolution logic depends on the resolveDir parameter including looking for packages in node_modules directories (since it needs to know where those node_modules directories might be).

• If you want to resolve a file name in a specific directory, make sure the input path starts with ./. Otherwise the input path will be treated as a package path instead of a relative path. This behavior is identical to esbuild's normal path resolution logic.

• If path resolution fails, the errors property on the returned object will be a non-empty array containing the error information. This function does not always throw an error when it fails. You need to check for errors after calling it.

• The behavior of this function depends on the build configuration. That's why it's a property of the build object instead of being a top-level API call. This also means you can't call it until all plugin setup functions have finished since these give plugins the opportunity to adjust the build configuration before it's frozen at the start of the build. So the new resolve function is going to be most useful inside your onResolve and/or onLoad callbacks.

• There is currently no attempt made to detect infinite path resolution loops. Calling resolve from within onResolve with the same parameters is almost certainly a bad idea.

Fixes #641
Fixes #1652