add option to clear variant before install

Currently, repeatedly doing a rez-build -i will reinstall over the old package. If you do things like rename python files, your install will stack up with older files since they are never deleted.

There should be a setting to have rez delete an installed package variant before installing into it.

Start by tracing the rez-build -i install path and how an installed package variant is selected. Define where the new setting belongs and verify that enabling it removes the existing variant contents before reinstalling, including the renamed-file case described here.

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