Move all cmake/Find*.cmake files to extra-cmake-modules and add that as a dependency

[eszlari]

https://invent.kde.org/frameworks/extra-cmake-modules

"While the driving force of this module is to reduce duplication in CMake scripts
across KDE software, it is intended to be useful for any software that uses the
CMake build system."

CC @jlskuz

[jlskuz]

It sounds like a good idea to me, but as a KDE developer I am of course very biased.

While it would be easy to use in the Kdenlive build scripts (because we use it already anyways). It will be an extra piece for Shotcut and co. That's why I don't want to push too hard on it and go with whatever @ddennedy decides.

Some things to take into consideration:

• ECM is lightweight in the sense that it does not have dependencies on other software
• ECM will be a build time dependency only
• If we upstream files to ECM it will take several months or years until a release including them is populated to all relevant distributions such as Ubuntu LTS. So this will be a long-term project.
• For FindFFmpeg.cmake there is already a merge request open
• Other features besides the find modules such as https://api.kde.org/ecm/kde-module/KDEGitCommitHooks.html might be useful for MLT too