formula_installer: reduce dependency expansion computations

[Bo98]

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• Cache compute_dependencies calls so we don't recompute at fetch and install time.
• Avoid compute_dependencies calls in the ignore_deps? case.

[BrewTestBot]

Review period will end on 2021-03-09 at 00:00:00 UTC.

[MikeMcQuaid]

Some style comments but 👍🏻, nice work.

[BrewTestBot]

Review period skipped due to critical label.