interpreter: add overload for source_strings_to_files
interpreter: add overload for source_strings_to_files

when only passing strings or Files we only get back Files. This is
useful for the install_* methods

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