I'm not aware that such a n extension exists. I would also rather not use it and I think that there's even no (easy/standard) way of getting the information required to login simply from the passwd file, even given that your system is not using a sahdow file for passwords (which it probably does). I'm not even sure, if you can read all the required files from php without root access, and if not, running php as a root user is probably a big security issue as well.
So, instead of looking for such a way, it's probably more beneficial to look in a better way to manage your users. Probable solutions would be to have an LDAP service, which MediaWiki can integrate with, using extensions (LDAPAuthentication2). Or you could find another user management system of your choice and integrate MediaWiki using OAuth or SAML.