I've reviewed this PR again and I feel like this pull request should be done on fish's github instead. Basically, only shell completions are being added here and fish already includes programs completions, could you open this PR there instead? Thanks.
The composer is prefered to be installed to global PATH but, some ppl prefer to have it localy for every project.
Some ppl rename composer.phar to only composer, so it needs to be handled.
In a way you need to handle every possible way.
I was just looking for a composer plugin. homebrew installs it globally links it and works great. My only thing is that the path needs amended to include ~/.composer/vendor/bin for composer executables. pretty easy fix.