Rails: Remove circular require

[marcotc]

What does this PR do?

This PR removes a few circular requires in the Rails integrations, like the following example:

from ddtrace/lib/datadog.rb:5:in  `<top (required)>'
  from ddtrace/lib/datadog.rb:5:in  `require_relative'
  from ddtrace/lib/datadog/tracing/contrib.rb:35:in  `<top (required)>'
from ddtrace/lib/datadog/tracing/contrib.rb:35:in  `require_relative'
  from ddtrace/lib/datadog/tracing/contrib/action_pack/integration.rb:6:in  `<top (required)>'
  from ddtrace/lib/datadog/tracing/contrib/action_pack/integration.rb:6:in  `require_relative'
from ddtrace/lib/datadog/tracing/contrib/rails/integration.rb:7:in  `<top (required)>'
  from ddtrace/lib/datadog/tracing/contrib/rails/integration.rb:7:in  `require_relative'
  from ddtrace/lib/datadog/tracing/contrib/rails/patcher.rb:5:in  `<top (required)>'
from ddtrace/lib/datadog/tracing/contrib/rails/patcher.rb:5:in  `require_relative'
  from ddtrace/lib/datadog/tracing/contrib/rails/framework.rb:8:in  `<top (required)>'
  from ddtrace/lib/datadog/tracing/contrib/rails/framework.rb:8:in  `require_relative'
from ddtrace/lib/datadog/tracing/contrib/active_support/integration.rb:7:in  `<top (required)>'
  from ddtrace/lib/datadog/tracing/contrib/active_support/integration.rb:7:in  `require_relative'

How to test the change?

There should be no change in behaviour as long as integration tests pass in CI.

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.11%. Comparing base (d872d68) to head (4c585a4).
Report is 8 commits behind head on 2.0.

Additional details and impacted files

@@           Coverage Diff           @@
##              2.0    #3567   +/-   ##
=======================================
  Coverage   98.11%   98.11%           
=======================================
  Files        1237     1237           
  Lines       72194    72194           
  Branches     3396     3396           
=======================================
+ Hits        70830    70832    +2     
+ Misses       1364     1362    -2     

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[ivoanjo]

Would it make sense to also backport this to the 1.x branch?