How to get the directory of a package?

[Marcin Borkowski]

Hello,

assume that I'm writing a package which wants to use a file from the
same directory the .el (or .elc) file of the package is located in.  How
can the package know where in the filesystem it is located?

Bonus points for a method which works not only with `require', but also
with `load-file' and `eval-buffer' (in the last case,
`default-directory' will probably work, but I skimmed through all
variables and functions matching `directory$' and nothing seemed to be
what I'm looking for).

TIA,

-- 
Marcin Borkowski
http://mbork.pl