Use sort_by to preserve user-defined order of package installs

[charlespierce]

Closes #478

Use sort_by and specify an ordering function to do the sorting of tools. That sorting function explicitly treats Node as the lowest, followed by Npm, Yarn, and Packages, but within each category it ignores the specifics and treats all tools as the same.

Per the rust docs, Vec::sort_by is stable, so by treating all of the Packages as equal, the original order (provided by the user) will be preserved.

Note: We use sort_by as opposed to a custom implementation of the Ord trait because the docs specify that the implementations of Ord and PartialEq must match exactly, and we use the derived PartialEq implementation elsewhere so it can't be changed.

[rwjblue]

Thank you for working on this! The current behavior confused me when I first noticed it.

[chriskrycho]

Thanks for this! One strongly-recommended tweak, and then a set of nits you can totally feel free to ignore. 😆

[chriskrycho]

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