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flutter: move build-support/flutter to flutter directory
Rationale: With the advent of RFC 140, Nixpkgs becomes to promote self-contained package directories and strongly discourages scattering along the old and deprecated filesystem hierarchy. Further, `build-support/flutter/*` is not referenced by any other expressions in Nixpkgs besides flutter itself. Further, concentrating all Flutter-related toolset below a same directory hierarchy (arguably?) eases maintenance and future migrations.