[rubygems/rubygems] Improve bundler/setup performance again

On a different patch, it was noticed Ngam Pham that we are calling
`LazySpecification#hash` many times, and simply memoizing that led to a
very considerable performance improvement in his app.

I noticed though that we shouldn't be calling `LazySpecification#hash`
that many times, and I located the culprit at `SpecSet#for` where we
were deduplicating the partial aggregated result on every iteration. It
is enough to do it just once at the end.

This leads on a 12% speedup on Rails repository Gemfile vs the previous
8% I was getting from memoizing `LazySpecification#hash`. Also, after
this patch memoizing `LazySpecification#hash` has no effect in
performance anymore.

https://github.com/rubygems/rubygems/commit/68d00a9edd

Co-authored-by: Ngan Pham <ngan@users.noreply.github.com>

lib/bundler/spec_set.rb

@@ -24,7 +24,7 @@ module Bundler
 
         specs_for_dep = spec_for_dependency(dep, match_current_platform)
         if specs_for_dep.any?
-          match_current_platform ? specs += specs_for_dep : specs |= specs_for_dep
+          specs += specs_for_dep
 
           specs_for_dep.first.dependencies.each do |d|
             next if d.type == :development
@@ -40,6 +40,8 @@ module Bundler
         specs << spec
       end
 
+      specs.uniq! unless match_current_platform
+
       check ? true : specs
     end