[gentoo-user] Gentoo package database

[ivan ivanov]

Hello, list. I am interesting how to find the package that contains the 
file that i want to have in my system.
Let's suppose that latex tells me: File upgreek.sty  not found. To solve 
this i need to install dev-texlive/texlive-latexextra. But, if i do not 
know this, i have to search the appropriate package. On debian i can 
easily do this by visiting official database 
http://www.debian.org/distrib/packages . On archlinux i can use "pkgfile 
upgreek.sty" that searches through NOT INSTALLED packages.
I found site http://www.portagefilelist.de/ that does this job for 
gentoo. But, as i suppose, it is third party project.

I cannot believe that such great project as gentoo does not have 
OFFICIAL database, that allows to search package by the file name.

So, the question is: how to find the name of NOT installed package that 
provides  needed file?