Allows drush make to completely overwrite existing directories.
Fix #874. Check module path is valid before adding it to $searchpath.
Merge pull request #873 from drush-ops/allow-overwrite

Allows drush make to completely overwrite existing directories.
Fix #886. Fatal error: Call to undefined function drush_module_exists() on test-run
Adds the creation time of the extension's .info file to projects. Similar to what update.module does.