mesonlib: Fix type annotations
mesonlib: Fix type annotations

get_compiler_for_source and classify_unity_sources are both wrong, in
that they expect to be given a seqence of strings, but they really
should take a `Sequence[str | File]`.

Additionally, they're using `CompilerType`, which we don't need anymore,
and should stop using, most methods for the Compiler are actually
defined in the base compiler class.

mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson