fix: add @ngtools/webpack to project deps only if @angular-devkit/build-angular is not a dependency

[sis0k0]

Revert to adding @ngtools/webpack instead of
@angular-devkit/build-angular because the later is ~150mb bigger and
slows down the {N} cloud builds.
Add @ngtools/webpack to the project dependencies only if
@angular-devkit/build-angular isn't there already. That's because the
@angular-devkit/build-angular already depends on @ngtools/webpack. This will prevent the
plugin from adding multiple instances of @ngtools/webpack to the project
which would cause the build to fail.

related to #571, #569

BREAKING CHANGES

Not really a breaking change but:
It's a good idea to remove the @angular-devkit/build-angular from the
package.json and add @ngtools/webpack instead as this will speed up
the build.

fixes #595

[vchimev]

test