…stic#24797)

I noticed the following message while watching the build output:

```
00:33:19.131    │ info [kibana] > /var/lib/jenkins/workspace/elastic+kibana+pull-request+multijob-x-pack/install/kibana/bin/kibana --optimize.useBundleCache=true --env.name=development --logging.json=false --server.port=5620 --optimize.watchPort=5630 --optimize.watchPrebuild=true --status.allowAnonymous=true --elasticsearch.url=http://elastic:changeme@localhost:9220 --elasticsearch.username=elastic --elasticsearch.password=changeme --server.uuid=5b2de169-2785-441b-ae8c-186a1936b17d --xpack.xpack_main.telemetry.enabled=false --xpack.security.encryptionKey="wuGNaIhoMpk5sO4UBxgr3NyW1sFcLgIf" --optimize.enabled=true --xpack.reporting.capture.browser.type=chromium --xpack.spaces.enabled=false
00:33:38.533    │ proc [kibana]   log   [23:20:25.923] [info][optimize] Optimizing and caching bundles for ml, stateSessionStorageRedirect, status_page, timelion, graph, monitoring, login, logout, dashboardViewer, apm, canvas, infra and kibana. This may take a few minutes
00:34:12.650    │ proc [kibana] [BABEL] Note: The code generator has deoptimised the styling of "/var/lib/jenkins/workspace/elastic+kibana+pull-request+multijob-x-pack/install/kibana/node_modules/x-pack/plugins/infra/node_modules/lodash/lodash.js" as it exceeds the max of "500KB".
00:36:11.346    │ proc [kibana]   log   [23:22:58.721] [info][optimize] Optimization of bundles for ml, stateSessionStorageRedirect, status_page, timelion, graph, monitoring, login, logout, dashboardViewer, apm, canvas, infra and kibana complete in 152.79 seconds
```

The "code generator has deoptimised" line specifically raised a red flag as babel shouldn't be running on node_modules. We have admittedly weak regular expressions in two places to enforce this. The first is in https://github.com/elastic/kibana/blob/49071132c3c14590dea877bccb7647bbec1e1459/src/setup_node_env/babel_register/register.js#L42, which I verified matched `node_modules` directories within the `node_modules/x-pack` directory, but the other is https://github.com/elastic/kibana/blob/49071132c3c14590dea877bccb7647bbec1e1459/src/optimize/base_optimizer.js#L152-L154 which does not. It only excludes files from babel in webpack if they are within a node_modules directory that is a direct child of `node_modules/xpack`, but with InfraOps and Canvas we now have node_module directories at `plugins/*/node_modules`.

This should probably be fixed by preventing plugins from installing their own node_modules, but since that would involve upgrading/moving plugins between major versions of dependencies like lodash that's somewhat impractical from where I stand, so instead I've just updated the webpack module rule to exclude any `node_modules/xpack/**/node_modules` directory.