Version Range in Variant breaks build

As discussed over IM, using a package variant like this `usd-0.8.0+<0.8.1` will break a build.

```
CMake Error at ./rezplugins/build_system/cmake_files/InstallPython.cmake:103
(add_custom_command)
add_custom_command called with OUTPUT containing a "<". This character is not allowed.
```

Start in rezplugins/build_system/cmake_files/InstallPython.cmake around line 103 and reproduce the failure with the `usd-0.8.0+<0.8.1` package variant. Trace how the variant becomes the CMake output name; done means the build no longer rejects the generated output because it contains `<`.

Written by the indexing model from the issue text.