Tomato, Tomahto. Tiny Core tends to do it's own thing for naming extensions, that's okay by me. They are not required to do what distribution abc or xyz does.
Open Apps, select 'provides' and enter 'mesa' as a search word. In a couple seconds you'll get results. Click on result then 'file' tab, you'll see Mesa files are included in Xorg-7.7-3d.tcz. Running Apps and searching 'tags' is also often useful, but not in this particular case.