Merge Finddouble-conversion.cmake contents into FindWrapDoubleConversion.cmake

Qt ships 2 Find modules for the double conversion library:
FindWrapDoubleConversion.cmake
Finddouble-conversion.cmake

This is troublesome if somebody who builds Qt wants to user their own Finddouble-conversion.cmake file, because the Qt one is always preferred.

One particular use case of this is building Qt with 3rd party dependencies provided by Conan, where Conan would generate its own Finddouble-conversion.cmake file.

To allow others to provide their own file, we should instead merge the contents of Finddouble-conversion.cmake into FindWrapDoubleConversion.cmake and only use that code if the 3rd party one could not be found.

This is a slight behavior change, and might cause some issues in case if pre-existing Finddouble-conversion files in the wild are broken (we've had multiple issues we had to work around with harfbuzz config files for example).

At the same time, for consistency, we could rename the file to FindWrapSystemDoubleConversion.cmake