Find datadog spec even when extensions are not built

`rubygems` started enforcing extensions to be built via
`Gem::Specification#ignored?`.

This causes the `datadog` spec lookup via
`Gem::Specification.find_by_name` to fail.

Since in this case we don't care for the extension, replace by
`Gem.loaded_specs`, which is what matters there.

Note that `Gem::SpecificationRecord.find_all_by_name` is not subject to
the `ignored?` check, only `Specification.find_by_name` is (via
`Gem::Dependency#to_spec`), so we could use that; unfortunately it is
fairly recent and unavailable for older rubygems versions.

See:

- ruby/rubygems@c80998a
- ruby/rubygems#8104
- #4511 (comment)

Author: lloeki

spec/datadog/tracing/contrib_spec.rb

@@ -1,7 +1,7 @@
 require 'spec_helper'
 
 RSpec.describe Datadog::Tracing::Contrib do
-  root = Gem::Specification.find_by_name('datadog').gem_dir
+  root = Gem.loaded_specs['datadog'].gem_dir
 
   # Our module names are camelized directory name with some exceptions
   mapping = {