Automatic fallback to cmake and cargo subproject

CMake subprojects have been supported for a while using the cmake.subproject() module method. However until now it was not possible to use a CMake subproject as fallback in a dependency() call.

A wrap file can now specify the method used to build it by setting the method key in the wrap file's first section. The method defaults to meson.

Supported methods:

• meson requires meson.build file.
• cmake requires CMakeLists.txt file. See details.
• cargo requires Cargo.toml file. See details.