Improve guessComposerPath
minor #37 Improve guessComposerPath (jeremy-derusse)

This PR was merged into the master branch.

Discussion
----------

Improve guessComposerPath

Given the `ExecutableFinder` can not search for a given extension (https://github.com/symfony/symfony/issues/12247).

I refactor the method `guessComposer` to search for executable with or without extension `.phar` and to also provide a command prefixed by PHP in case of file `composer` exists and is not executable.

Commits
-------

9db380d Improve guessComposerPath