feat(@schematics/angular): use es2015 modules overall

[filipesilva]

This change helps to keep the behaviour of build/serve/test consistent by using the same import semantics in all of them.

[mgol]

Something to check: what about tsconfig.server.json when Universal is used? Will it still work correctly with "module": "es2015"?

[filipesilva]

@mgol good catch, added now.

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