Fix PEAR.php loading for Composer custom installers

[greg-1-anderson]

Do not try to call require_once on PEAR.php if it has already been loaded by the autoloader.

Motivation

The require_once of PEAR.php can cause a fatal error if this file is not available on the PHP include path. When using an autoloader, such as with Composer, this file does not need to be loaded; we can avoid needlessly stopping execution simply by testing to see if the file is already available via the autoloader.

Note that for a typical Composer application, the require line does not cause any problems, as Archive_Tar has an include line in its composer.json that allows PHP to find it when requested. However, Composer custom installers cannot benefit from this, because include and autoload files are not available in custom installers. Inside custom installers, Archive_Tar will fail if PEAR.php is not installed on the global PHP include path.

For more information, see drupal-composer/drupal-project#111

[greg-1-anderson]

If you accept this PR, it would also be very helpful if you could tag a bugfix release so that this fix is available to Windows Composer custom installer users.

[mrook]

Thanks for the PR, I'll tag a release.

[greg-1-anderson]

Thanks so much!