Refactor PackageManagers

[ryanmrichard]

Right now the PackageManger API is comprised of three members:

• find_installed,
• get_package, and
• install_package

In both the CMakePackageManager and the PIPPackageManger the find_installed looks for an already installed package, returning nothing if it's not found. get_package for CMakePackageManager will call FetchContent whereas get_package for PIPPackageManager wraps find_installed and raises an error if the package is not already installed. CMakePackageManger implements install_package by wrapping CMake's install commands whereas PIPPackageManager wraps pip's install command. The package managers should be implemented in a consistent manner (probably want to change the behavior of the PIPPackageManager to be more like the CMakePackageManager).