[rubygems/rubygems] Improve performance of Bundler::SpecSet#for by using hash lookup of handled deps

I was looking at (yet another) flamegraph in speedscope, and used the
'left hand heavy' and was shocked to realize that 0.5s of the 1.7s
is spent in DepProxy#name. This method _only_ delegates the name to an
underlying spec, so it's not complex at all.

It seems to be of how often this line ends up calling it:

     next if handled.any?{|d| d.name == dep.name && (match_current_platform || d.__platform == dep.__platform) } || dep.name == "bundler"

The `handled` array is built up as dependencies are handled, so this get
slower as more dependencies are installed.

This change changes how `handled` is track. Instead of just an array, I've
tried using a Hash, with the key being a dep's name, and the value being
a list of deps with that name. This means it's constant time to find
the dependencies with the same name.

I saw a drop from 1.7s to 1.0s against master, and from 0.95s to 0.24s
when used with https://github.com/rubygems/rubygems/pull/5533

https://github.com/rubygems/rubygems/commit/844dac30d4

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