Change exit code on empty requirements and fix false positive on php requirement

[icanhazstring]

When there are no packages to scan then there should
be no need to exit the process with an exit code larger than 0.

This should solve a problem for any CI integration for new packages
where composer-unused will be installed and used from the get go.

Solve #78

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Also fixed an issue where Core extension was checked as ext-core instead of php
Solve #79

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