This change first attempts to resolve a schematic referenced via the external schematic rule from the requesting schematic collection. This allows schematic packages that are direct dependencies of another schematic package to be used with the external schematic rule without manual package resolution code within the schematic.
Closes#18098Closes#11026
With Angular CLI version 8, migrations cannot be re-run with the
`--migrate-only` flag as there was a recent regression introduced
in e406f00909 (diff-0d0a748fb9a38a7ccde08d9b42e70bce) as it now passes
a normalized platform path to the `engine.createCollection` call.
This breaks as there is incorrect logic within
`node-modules-engine-host` that causes the schematic collection to be
searched within the `package.json#schematics` entry. This is incorrect
as migration schematics specify their migration schematics in a separate
schematic collection file which is part of `package.json#ng-update`.
Fixes#14565
