structured_sources() is now checked and copies files when needed

A bugs in structured_sources() has been fixed which can cause a Rust target to be built from a different set of files than before. In particular, the check to detect whether source files have to be copied to the build directory was previously wrong, and calls that only move around source files require Meson 1.13.0 or newer. For example:

structured_sources(['main.rs'], {'foo': 'linux/foo.rs'})

did not place linux/foo.rs in foo.rs, and mod foo; failed to compile. The layout is now the same whether the structure contains generated files or not, and files are copied requested layout differs from the layout in the source directory.