Don't discard candidates matching Ruby metadata

[deivid-rodriguez]

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

Sometimes a Gemfile.lock file will include a variant that matches the current Ruby, but Bundler will discard it because of not being the most platform specific one.

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

Bundler materializes lockfiles into real specifications in two steps. First, it filters the dependencies it needs (for example, if BUNDLE_WITHOUT is set or things like that), and chooses a set of specs from those. Then it converts those lockfile specifications into real specifications.

From these two steps, only the second one considered Ruby metadata, while both steps considered platform. This resulted on these candidates already discarded by the time we try to materialize specifications.

This PR merges these two steps into a single pass, so that we consider metadata and platforms at the same time. This also speeds up require "bundler/setup" time in our example big Gemfile by about 2-3%.

✗ hyperfine 'BUNDLER_VERSION=2.4.0.dev ruby -rbundler/setup -e1' 'BUNDLER_VERSION=2.3.19 ruby -rbundler/setup -e1'  
Benchmark 1: BUNDLER_VERSION=2.4.0.dev ruby -rbundler/setup -e1
  Time (mean ± σ):     135.5 ms ±   1.0 ms    [User: 112.1 ms, System: 22.1 ms]
  Range (min … max):   134.2 ms … 138.4 ms    21 runs
 
Benchmark 2: BUNDLER_VERSION=2.3.19 ruby -rbundler/setup -e1
  Time (mean ± σ):     138.9 ms ±   0.5 ms    [User: 115.6 ms, System: 21.9 ms]
  Range (min … max):   138.1 ms … 139.9 ms    21 runs
 
Summary
  'BUNDLER_VERSION=2.4.0.dev ruby -rbundler/setup -e1' ran
    1.03 ± 0.01 times faster than 'BUNDLER_VERSION=2.3.19 ruby -rbundler/setup -e1'

Closes #5715.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
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[deivid-rodriguez]

As a side effect, this PR also implements a different improvement, namely, auto fixing lockfiles generated with different Ruby versions automatically, instead of raising errors. That, of course, unless we are in frozen mode and the lockfile can't be modified. This improvement was already planed since #5435 (comment).