Error building jupyterlab

I'm having trouble getting an end to end build to work with the current tree.

Everything is OK through to the last command but then this command:

```
jupyter lab build
```

fails with:

```
[LabBuildApp] > node /data/tools/anaconda3/envs/beakerx_again/lib/python3.7/site-packages/jupyterlab/staging/yarn.js run build:dev:minimize
[LabBuildApp] yarn run v1.15.2
$ jlpm run build:dev
$ jlpm run build
$ webpack
ModuleNotFoundError: Module not found: Error: Can't resolve '../lib/tree.js' in '/Volumes/Space2/tools/anaconda3/envs/beakerx_again/share/jupyter/lab/staging/node_modules/beakerx-jupyterlab/dist'
at /Volumes/Space2/tools/anaconda3/envs/beakerx_again/share/jupyter/lab/staging/node_modules/webpack/lib/Compilation.js:925:10
at /Volumes/Space2/tools/anaconda3/envs/beakerx_again/share/jupyter/lab/staging/node_modules/webpack/lib/NormalModuleFactory.js:401:22
at /Volumes/Space2/tools/anaconda3/envs/beakerx_again/share/jupyter/lab/staging/node_modules/webpack/lib/NormalModuleFactory.js:130:21
at /Volumes/Space2/tools/anaconda3/envs/beakerx_again/share/jupyter/lab/staging/node_modules/webpack/lib/NormalModuleFactory.js:224:22
...
```

I got it to go a little further by doing this:

```
cd /share/jupyter/lab/staging/node_modules/beakerx-jupyterlab/dist
mkdir ../lib
cp -R * ../lib/
```
(this just copies the files it's looking for to the place that it is looking for them).

However then that just dies with a similar error but looking for a file that is nowhere in the source tree:

```
ModuleNotFoundError: Module not found: Error: Can't resolve '../lib/td_index.js' in '/Volumes/Space2/tools/anaconda3/envs/beakerx_again/share/jupyter/lab/staging/node_modules/beakerx-jupyterlab/dist'
```

Any hints to get the build working?

Start by reproducing the reported end-to-end build with `jupyter lab build`, then inspect the `beakerx-jupyterlab` package under `staging/node_modules`, especially its `dist` references to `../lib/tree.js` and `../lib/td_index.js`. Done means the build completes without missing-module errors; the report does not identify a source file or intended fix.

Written by the indexing model from the issue text.