build: wrap bazel-built tars in a package folder

[kormide]

@josephperrott @gregmagolan

The tars produced by the current build script enclose the tar contents in a package folder. See https://github.com/angular/angular-cli/blob/master/scripts/build.ts#L76.

Is this something you want to preserve?

It turns out that the tar functionality built into pkg_npm already does this by default so we can just use that instead of pkg_tar.

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