Did you resolve it?
Got the same issue after I just tried building & running the android mobile app, fresh clone after an upstream fetch:

info Reloading app...
 BUNDLE  ./index.js 

error: Error: Unable to resolve module @joplin/fork-uslug from /home/dae/.local/share/Trash/files/joplin/packages/renderer/MdToHtml.js: @joplin/fork-uslug could not be found within the project or in these directories:
  ../renderer/node_modules
  ../../node_modules
  ../../../../../../../node_modules
  /home/dae/.local/share/Trash/files/joplin/packages/app-mobile/node_modules/@joplin/fork-uslug

If you are sure the module exists, try these steps:
 1. Clear watchman watches: watchman watch-del-all
 2. Delete node_modules and run yarn install
 3. Reset Metro's cache: yarn start --reset-cache
 4. Remove the cache: rm -rf /tmp/metro-*
  34 |     source_map: require('./MdToHtml/rules/source_map').default,
  35 | };
> 36 | const uslug = require('@joplin/fork-uslug');
     |                        ^
  37 | const markdownItAnchor = require('markdown-it-anchor');
  38 | // The keys must match the corresponding entry in Setting.js
  39 | const plugins = {
    at ModuleResolver.resolveDependency (/home/dae/.local/share/Trash/files/joplin/packages/app-mobile/node_modules/metro/src/node-haste/DependencyGraph/ModuleResolution.js:129:15)
    at DependencyGraph.resolveDependency (/home/dae/.local/share/Trash/files/joplin/packages/app-mobile/node_modules/metro/src/node-haste/DependencyGraph.js:288:43)
    at Object.resolve (/home/dae/.local/share/Trash/files/joplin/packages/app-mobile/node_modules/metro/src/lib/transformHelpers.js:129:24)
    at resolve (/home/dae/.local/share/Trash/files/joplin/packages/app-mobile/node_modules/metro/src/DeltaBundler/traverseDependencies.js:396:33)
    at /home/dae/.local/share/Trash/files/joplin/packages/app-mobile/node_modules/metro/src/DeltaBundler/traverseDependencies.js:412:26
    at Array.reduce (<anonymous>)
    at resolveDependencies (/home/dae/.local/share/Trash/files/joplin/packages/app-mobile/node_modules/metro/src/DeltaBundler/traverseDependencies.js:411:33)
    at processModule (/home/dae/.local/share/Trash/files/joplin/packages/app-mobile/node_modules/metro/src/DeltaBundler/traverseDependencies.js:140:31)
    at runMicrotasks (<anonymous>)
    at processTicksAndRejections (internal/process/task_queues.js:95:5)