Improve bundled gems warnings for subfeatures

Before, when requiring "bigdecimal/math" in a Bundler context:

> /Users/deivid/.asdf/installs/ruby/3.3.0-dev/lib/ruby/3.3.0+0/bigdecimal/math.rb:2: warning: bigdecimal was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.4.0. Add bigdecimal to your Gemfile or gemspec.

After:

> foo.rb:1: warning: bigdecimal/math is found in bigdecimal, which will no longer be part of the default gems since Ruby 3.4.0. Add bigdecimal to your Gemfile or gemspec.

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