Remove Support for Fetching PackageProject.cmake from CPM.cmake

[threeal]

Problem:

• There's no need to fetch PackageProject.cmake from CPM.cmake if it's already vendored within this project.

Solution:

• Remove the functionality to fetch PackageProject.cmake from CPM.cmake and use the vendored PackageProject.cmake instead.
• Remove the CPM.cmake module.
• Implement proper handling for the Boost::ut alias declaration (see also).

Issue: #631.

Reviewers:
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