Helper function for Drush only checks for a directory, not the actual JavaScript file

While resolving #1675964: Helper function for Drush command checks the wrong path I realized that the function only checks the directory that should contain the JS library instead of checking for the library itself. To reproduce:

$ drush dl modernizr libraries -y
$ drush en libraries modernizr -y
  [it will say it downloaded the library to /sites/all/libraries/modernizr]
$ rm -f /sites/all/libraries/modernizr/modernizr*
$ drush en modernizr
  [it will say the library is already present, but we just deleted it!]

This isn't critical because you'd have to go out of your way to cause this situation. Normal use of the module will never surface this bug.