Compatibility check for extensions not working

[jasongrout]

Description

It seems that the compatibility check for versions is not working, when it at least should give a warning.

Reproduce

conda create -n jlabversion notebook nodejs
conda activate jlabversion
pip install --pre jupyterlab==2.0.0a0

Now try installing the widgets extension.

jupyter labextension install @jupyter-widgets/jupyterlab-manager

I think because of the yarn resolutions, it no longer fails to install. This is good (in that we can test the widgets extension in an alpha version easily) and bad (there should be at least some warning that we are installing into a jlab where the versions conflict with what is requested).

Expected behavior

I think the version compatibility check should at least warn if we are installing into a jlab prerelease where the eventual non-prerelease would conflict, and it should error out and not let you install the incompatible extension if jlab is not a prerelease version.

In this case, I would expect a warning that the extension is not compatible with the eventual release, but we are letting you install anyway, and I would expect installing into 2.0 final release would fail.

Context

• Operating System and version: macOS 10.14.6
• Browser and version: N/A
• JupyterLab version: 2.0.0a0