CPack: Restore support for custom package configuration templates

Refactoring in commit 98617f1b (Refactor: Move CPack internal files to
`Internal/CPack/` directory, 2019-07-09, v3.16.0-rc1~449^2) accidentally
changed the public-facing names of the templates.  The name passed to
`FindTemplate` is searched in `CMAKE_MODULE_PATH` and should not change.
Remove the `Internal/CPack/` prefix on the names added by that commit.
Teach `FindTemplate` to use our builtin default directly when the
public-facing name is not found in `CMAKE_MODULE_PATH`.

Fixes: #19979
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