Prefix all our vendor libs #16905

see https://github.com/matomo-org/wp-matomo/issues/233

Need to do this in a major release. Should investigate if we can make this work. We'd only prefix vendor libraries. Could use Matomo or MatomoLib or something as prefix. eg Di\object then becomes Matomo\Di\object. To have it actually useful in development too we would put this into the composer json similar to https://github.com/google/site-kit-wp/issues/612 see eg https://github.com/google/site-kit-wp/pull/730/files

More details in #233