Missing dependency on preact / react (ghost dependency)

This module tries to import / require from `preact`, but does not declare it as a dependency in its `package.json` (a [ghost dependency](https://yarnpkg.com/features/pnp#ghost-dependencies-protection)). This is an error, [yarn requires all dependency to be listed properly](https://yarnpkg.com/configuration/yarnrc#packageExtensions):

```
Module not found: Error: Can't resolve 'preact' in 'C:\Users\user\AppData\Local\Yarn\Berry\cache\htm-npm-3.1.1-e3b831f850-10.zip\node_modules\htm\preact'
resolve 'preact' in 'C:\Users\user\AppData\Local\Yarn\Berry\cache\htm-npm-3.1.1-e3b831f850-10.zip\node_modules\htm\preact'
Parsed request is a module
using description file: C:\Users\user\AppData\Local\Yarn\Berry\cache\htm-npm-3.1.1-e3b831f850-10.zip\node_modules\htm\preact\package.json (relative path: .)
Field 'browser' doesn't contain a valid alias configuration
resolve as module
request is not managed by the pnpapi
htm tried to access preact, but it isn't declared in its dependencies; this makes the require call ambiguous and unsound.
Required package: preact
Required by: htm@npm:3.1.1 (via C:\Users\user\AppData\Local\Yarn\Berry\cache\htm-npm-3.1.1-e3b831f850-10.zip\node_modules\htm\preact\)
@ ../../../../../../AppData/Local/Yarn/Berry/cache/@bpmn-io-diagram-js-ui-npm-0.2.3-01795dd8e0-10.zip/node_modules/@bpmn-io/diagram-js-ui/lib/index.js 1:0-34 1:0-34
@ ../../../../../../AppData/Local/Yarn/Berry/cache/diagram-js-npm-15.2.4-de7db86f90-10.zip/node_modules/diagram-js/lib/ui/index.js 1:0-39 1:0-39
@ ../../../../../../AppData/Local/Yarn/Berry/cache/diagram-js-npm-15.2.4-de7db86f90-10.zip/node_modules/diagram-js/lib/features/popup-menu/PopupMenu.js 1:0-4:18 132:2-8 133:4-8 278:2-8
@ ../../../../../../AppData/Local/Yarn/Berry/cache/diagram-js-npm-15.2.4-de7db86f90-10.zip/node_modules/diagram-js/lib/features/popup-menu/index.js 1:0-36 12:23-32
@ ../../../../../../AppData/Local/Yarn/Berry/cache/bpmn-js-npm-18.2.0-5474b88486-10.zip/node_modules/bpmn-js/lib/features/align-elements/index.js 3:0-65 13:4-19
@ ../../../../../../AppData/Local/Yarn/Berry/cache/bpmn-js-npm-18.2.0-5474b88486-10.zip/node_modules/bpmn-js/lib/Modeler.js 12:0-60 167:2-21
@ ./src/bpmn-editor/render-bpmn-editor.ts 7:0-48 33:31-41
@ ./src/bpmn-editor/index.ts 1:0-40 1:0-40
```

The same goes for `react`. Possible ways how the dependencies could be fixed:

* declare both react and preact as dependencies (will always install both preact and react)
* declare both react and preact as optional peer dependencies (preferred) ~~(will force users to install both react and preact)~~
* ~~create 2 separate NPM packages, so that each package can declare its dependencies properly~~

# Workaround

For people affected: You can add the following [bandage aid](https://yarnpkg.com/configuration/yarnrc#packageExtensions) to fix the declared dependencies (file `.yarnrc.yml`):

```yaml
packageExtensions:
htm@*:
peerDependencies:
preact: "*"
react: "*"
peerDependenciesMeta:
preact:
optional: true
react:
optional: true
```

Inspect the package.json dependency declarations and the preact and react import paths described in the report. Reproduce the failure with Yarn Plug'n'Play, then compare the dependency options in the issue. Done means the package declares its runtime requirements correctly and no ghost-dependency error occurs for either supported library.

Written by the indexing model from the issue text.