Add resolveDir into OnResolveResult and OnLoadArgs

Currently we have 2 hooks `onResolve()` and `onLoad()`, both have mostly symmetric input and output. However, `resolveDir` is respecting in result of `onLoad()` (`OnLoadResult`) only and using as input for `onResolve()` (`OnResolveArgs`). My suggestion is to respect `resolveDir` in `OnResolveResult` and pass it as `OnLoadArgs` into `onLoad()` too. `resolveDir` should be optional for `OnResolveResult` and be computed when omitted as `path.dirname(result.path)`.

This change may help to simplify custom loading a bit:

```js
onLoad({ ... }, args => ({
resolveDir: path.dirname(args.path),
contents: ...
}));
// ->
onLoad({ ... }, args => ({
resolveDir: args.resolveDir,
contents: ...
}));
```

It's much more useful when you need to specify `resolveDir` on resolving that differs from a path. Like in my case, when I use some common (generated) files for modules, but resolve imports depending on a module. Since `onResolve()` doesn't respect `resolveDir`, instead of:

```js
onResolve({ filter: /.../ }, args => ({
resolveDir: somethingFromPath(args.path),
path: 'path/to/common/file'
}));
```

I need to do the following (as workaround):

```js
onResolve({ filter: /.../ }, args => ({
namespace: 'common-file-example',
path: somethingFromPath(args.path)
}));
onLoad({ namespace: 'common-file-example', filter: /.*/ }, args => ({
resolveDir: args.path,
contents: fs.readFileSync('path/to/common/file')
}));
```

Personally, I prefer to avoid overloading loading behaviour as much as possible, because I believe `esbuild` makes it better. Such a change will avoid the need to do this.

Start from the onResolve() and onLoad() plugin API entry points and trace how OnResolveResult, OnLoadArgs, and resolveDir move through module resolution and loading. Confirm the existing fallback from an omitted OnResolveResult.resolveDir, pass the resolved directory into onLoad(), and verify the documented examples and behavior for both hooks.

Written by the indexing model from the issue text.