Dev installation instructions incur an error

[jasongrout]

Description

jlpm run build on a clean pip install prints out an error on current master (023828a)

Reproduce

1. pip install -ve .
2. jlpm run build

Gives an error:

(node:35285) UnhandledPromiseRejectionWarning: Error: Cannot find module 'react-json-tree/package.json'
Require stack:
- /[snip]/jupyterlab/buildutils/lib/utils.js
- /[snip]/jupyterlab/buildutils/lib/ensure-repo.js
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:713:15)
    at Function.Module._load (internal/modules/cjs/loader.js:618:27)
    at Module.require (internal/modules/cjs/loader.js:771:19)
    at require (internal/modules/cjs/helpers.js:68:18)
    at requirePackage (/[snip]/jupyterlab/buildutils/lib/utils.js:305:20)
    at /[snip]/jupyterlab/buildutils/lib/utils.js:280:31
    at Array.forEach (<anonymous>)
    at /[snip]/jupyterlab/buildutils/lib/utils.js:271:27
    at Array.forEach (<anonymous>)
    at Object.getPackageGraph (/[snip]/jupyterlab/buildutils/lib/utils.js:267:25)

This stems from a couple of things:

1. react-json-tree is for some reason not hoisted into the root node_modules, but instead lives in ./packages/json-extension/node_modules/react-json-tree
2. The test at

   jupyterlab/buildutils/src/utils.ts

   Line 321 in 023828a

   parentModulePath = require.resolve(parentModule);

   fails initially because Error: Cannot find module '/[snip]/jupyterlab/node_modules/@jupyterlab/json-extension/lib/index.js'. Please verify that the package.json has a valid "main" entry, so it falls through to the catch. However, it really failed because the json-extension package wasn't built yet, so the lib/ directory didn't exist.

This error goes away once the json-extension package is built, i.e., the second time jlpm run build is run.

I'm not sure why react-json-tree is not hoisted to the root node_modules.