module to check @INC only once for wanted modules

 Sometimes you want to lazy load a module for use in, say, a loop or function.
 First you do the eval-require but then realize if the module is not available
 it will re-search @INC each time. So then you add a lexical boolean to your
 eval and do the same simple logic all over the place.
 .
 Module::Want encapsulates that logic so that have_mod() is like eval {
 require X; 1 } but if the module can't be loaded it will remember that fact
 and not look in @INC again on subsequent calls.