Further speed up Bundler by not sorting specs unnecessarily

[deivid-rodriguez]

What was the end-user or developer problem that led to this PR?

Trying to make Bundler faster and simplify code at the same time.

What is your fix for the problem, implemented in this PR?

Improve bundler/setup time by ~2% in the big Gemfile I commonly use for testing performance (https://github.com/technicalpickles/big-gemfile), although it does not improve simpler setups like a brand new Rails app.

✗ hyperfine 'BUNDLER_VERSION=2.3.22 ruby -rbundler/setup -e1' 'BUNDLER_VERSION=2.4.0.dev ruby -rbundler/setup -e1'  --warmup 3
Benchmark 1: BUNDLER_VERSION=2.3.22 ruby -rbundler/setup -e1
  Time (mean ± σ):     141.4 ms ±   1.0 ms    [User: 112.5 ms, System: 27.6 ms]
  Range (min … max):   139.9 ms … 143.1 ms    20 runs
 
Benchmark 2: BUNDLER_VERSION=2.4.0.dev ruby -rbundler/setup -e1
  Time (mean ± σ):     139.0 ms ±   2.4 ms    [User: 109.4 ms, System: 27.6 ms]
  Range (min … max):   136.8 ms … 147.6 ms    21 runs
 
Summary
  'BUNDLER_VERSION=2.4.0.dev ruby -rbundler/setup -e1' ran
    1.02 ± 0.02 times faster than 'BUNDLER_VERSION=2.3.22 ruby -rbundler/setup -e1'

The main idea is to not sort specs unnecessarily when not needed although there are other misc improvements in the PR.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags