fix(installer): tarball installer checks for ini in mods-available

[ZNeumann]

Debian uses a directory called "mods-available" to house ini files. It's phpenmod and phpdismod allow command-line management of extensions in directory. It also creates a symlink ini file to this "mods-available" directory in the normal conf.d directory. The issue is that this symlink is prefixed with a number (for example 20-newrelic.ini). This prefix prevents our tarball installer from recognizing that a newrelic.ini already exists, resulting in 2 ini files: a newrelic.ini and a 20-newrelic.ini.

Our .deb package installer scouts out the "mods-available" directory and installs therein when possible. That means that if a customer installs via package but upgrades via tarball (not recommended), then their system gets the duplicates.

This change makes our tarball installer scout for a potential previous installation in mods-available.

Fixes #399

[newrelic-php-agent-bot]

Test Suite	Status	Result
Multiverse	✅	7/7 passing
SOAK	❌	54/56 passing

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.54%. Comparing base (f9530ed) to head (faa309e).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #971   +/-   ##
=======================================
  Coverage   78.54%   78.54%           
=======================================
  Files         196      196           
  Lines       27103    27103           
=======================================
  Hits        21287    21287           
  Misses       5816     5816           

Flag	Coverage Δ
agent-for-php-7.2	78.55% <ø> (ø)
agent-for-php-7.3	78.57% <ø> (ø)
agent-for-php-7.4	78.27% <ø> (ø)
agent-for-php-8.0	78.29% <ø> (ø)
agent-for-php-8.1	78.28% <ø> (ø)
agent-for-php-8.2	77.87% <ø> (ø)
agent-for-php-8.3	77.87% <ø> (ø)

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