fix(@angular-devkit/schematics): export task executor

[qiyigg]

No description provided.

[clydin]

Not being imported is intentional. Those are only relevant to node environments.

[qiyigg]

@clydin My understanding is: Those are executors, which are necessary for external workflow. Especially, BuiltinTaskExecutor.RunSchematic should be useful for all workflows. If you'd prefer access the executors through tasks/node/index.ts, I'd suggest reverting the module_root change here(https://github.com/angular/angular-cli/blob/master/packages/angular_devkit/schematics/BUILD#L74)

[qiyigg]

This was done by #12008
Close this PR right now.

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