Also extend our integration test double inclusion. It will still not
catch this case, because that requires
`cmake_minimum_required(VERSION 3.18)` or higher.
Fixes:
```
CMake Error at .../lib/cmake/libssh2/libssh2-config.cmake:8 (add_library):
add_library cannot create ALIAS target "libssh2::libssh2" because another
target with the same name already exists.
Call Stack (most recent call first):
CMakeLists.txt:24 (find_package)
CMake Error at .../lib/cmake/libssh2/libssh2-config.cmake:13 (add_library):
add_library cannot create ALIAS target "Libssh2::libssh2" because another
target with the same name already exists.
Call Stack (most recent call first):
CMakeLists.txt:24 (find_package)
```
Test to reproduce:
```cmake
cmake_minimum_required(VERSION 3.18) # must be 3.18 or higher
project(test)
find_package(libssh2 CONFIG)
find_package(libssh2 CONFIG) # fails
add_executable(test main.c)
target_link_libraries(test libssh2::libssh2)
```
Ref: https://cmake.org/cmake/help/latest/release/3.18.html#other-changes
Ref: https://cmake.org/cmake/help/v3.18/policy/CMP0107.html
Assisted-by: Kai Pastor
Assisted-by: Harry Mallon
Ref: https://github.com/curl/curl/pull/11913Closes#1216
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