Instead of doing automatic uninstalls in advance, install conflicting

Instead of doing automatic uninstalls in advance, install conflicting
packages first and then do the uninstall afterwards. This requires
special handling for file collisions occur, but it's preferred
because it ensures that package files remain installed in a usable
state whenever possible.

When file collisions occur between conflicting packages, the contents
entries for those files are removed from the packages that are
scheduled for uninstallation. This prevents uninstallation operations
from removing overlapping files that have been claimed by conflicting
packages.

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