If a package has a jslib or c++ plugin, Unity fails when building for WebGL.

The jslib path points to Package/* instead of Library/Package@version/*.
1. Create a new project.
2. Open Package Manger, add Analytics Library package.
3. Switch to WebGL in the Build Settings.
4. Build, and it will fail during the build process.

C++ plugins fail because the cpp file is copied to the Library folder, but headers are not, and this causes the headers to not be found during compilation.