formula_installer: pre-install implicit dependencies

[EricFromCanada]

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A leftover from #15566: originally added in #12296, a dependency with [:build, :test] was assumed to be required for downloading or extracting a package's source. Now we have :implicit, so just check for that. This restores curl being pre-installed for any dependency with using: :homebrew_curl and subversion being pre-installed if needed on macOS 10.14 and earlier.

[MikeMcQuaid]

Thanks @EricFromCanada!