Use FirePHP installed from PEAR if available

Problem/Motivation

Currently, the devel module installs and uses its own FirePHP library located in the Drupal libraries path. If FirePHP is already installed using PEAR, it would be preferable to use that instead. I manage my own FirePHP (and Drush) library installations using PEAR and would prefer if they were used over the versions found in the Drupal libraries path.

Proposed resolution

During initialisation of the devel module, a check should be done to see if FirePHP is already available in the default PEAR installation path. This PEAR library path is already added to the PHP include path, so the FirePHP PEAR library can always be found in the FirePHPCore subdirectory of that PEAR library path.

Remaining tasks

1. Update the Drush pm-download code so that if a PEAR install of FirePHPCore is found, the library isn't downloaded and added to the Drupal libraries path too.
2. It may be helpful to document this somewhere so that users understand which FirePHP library will be used, in order of precedence, if more than one exists on a system.