fix(agent): Fixes Doctrine 2.18+ detection

[mfulb]

Addresses changes in Doctrine 2.18+ (but not 3.x) that affected detection.

There was a commit for Doctrine 2.18 that reworked the directory structure of the Doctrine library. This PR adds another file for Doctrine to allow it to be auto-detected.

[newrelic-php-agent-bot]

Test Suite	Status	Result
Multiverse	✅	9/9 passing
SOAK	✅	56/56 passing

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.79%. Comparing base (bc0766b) to head (f89a63d).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #916   +/-   ##
=======================================
  Coverage   78.79%   78.79%           
=======================================
  Files         193      193           
  Lines       27214    27214           
=======================================
  Hits        21443    21443           
  Misses       5771     5771           

Flag	Coverage Δ
agent-for-php-7.0	77.53% <ø> (ø)
agent-for-php-7.1	77.27% <ø> (ø)
agent-for-php-7.2	78.20% <ø> (ø)
agent-for-php-7.3	78.22% <ø> (ø)
agent-for-php-7.4	77.93% <ø> (ø)
agent-for-php-8.0	77.99% <ø> (ø)
agent-for-php-8.1	77.98% <ø> (ø)
agent-for-php-8.2	77.58% <ø> (ø)
agent-for-php-8.3	77.58% <ø> (ø)

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[zsistla]

Thanks for investigating/fixing!