feat(@angular/build): Support Sass package importers #737

Job	Run time
list	26s
workflow (build-cli)	1m 42s
workflow (modules-builder-tests)	1m 49s
workflow (devkit-core-tests)	1m 27s
workflow (devkit-core-tests-rerun)	1m 19s
workflow (build-unit-tests)	2m 16s
	8m 59s

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