[amber][pkg_resolver][pkg_cache] Refactor BUILD.gn

This change modifies the package() template parameters for amber,
amber_tools, pkg_resolver, and pkg_cache to first invoke one or more
fuchsia_component templates and then aggregate those components into the
final packages.  This change is a no-op for now, but it will allow later
test cases to also aggregate these components into the test package.

PKG-604 #comment extract fuchsia_component templates

Change-Id: I3dae02ffd33ee410962ed622e6e664aaf309923c