Use case-insensitive search when matching package names

Composer matches all package names in a case insensitive manner, and so this PR makes sure we analyze the dependencies similarly.

After your recent changes to accommodate for mixed case of php extensions, this PR also applies a similar scheme to match all package names in case insensitive manner.

I'm not perfectly happy with the code quality of the outcome and can potentially see the need for a later refactoring, but I figured it's still a good idea to get some feedback on the progress. 

Refs clue/graph-composer#13