Filters out any x-pack workspaces from OSS builds. Fixes an issue introduced by #22618 that was causing the Infra plugin code to be included in the node_modules for OSS builds.
There are several cases where we want to do different things in the
optimizer based on whether we are running in the distributable version
of Kibana or running from source. Historically we have handled this by
relying on the env, but as we start to prebuild things, like TypeScript
and soon SCSS, we actually need to know if the version of Kibana running
was a result of the build process or not. To address this the src/utils
module now exports a constant: `IS_KIBANA_DISTRIBUTABLE`. This constant
will be based on the build.distributable flag being set in Kibana's
package.json, which is done automatically in the build process.
In order to make the license that applies to each file as clear as possible, and to be consistent with elasticsearch, we are adding Apache 2.0 license headers to the top of each file.
Existence of this header is enforced by eslint and tslint and missing headers were automatically added in the last commit by running:
```
node scripts/eslint --fix && node scripts/tslint --fix
```
