There was no reply within last 11 days neither here, nor on ubuntu forums. This is very disappointing.
I expected Ubuntu community to be more knowledgeable/experienced, since the problem isn't incredibly difficult, although it effectively disabled entire automatic update system. So much for being "user-friendly", I guess.
Anyway, I was able to fix this problem.
In situation like this (when you have broken dependency, and installing missing dependency is not possible), the only solution is to create virtual package (which is also called "dummy package" or "meta package") - i.e. empty package that tells system that it provides missing dependency.
In my case (required libldap2 >= 2.1.17-1):
1) go to any directory
2) mkdir libldap2/DEBIAN
3) vim libldap2/DEBIAN/control . Or use any other texteditor.
4) in file enter text:
Description: libldap2 dummy package
This package provides dpkg with the information that
there is a libldap2 installed
syntax of "control" file is explained somewhere on the web or in manpages.
5) save file, exit vim (or any text editor).
6) "sudo dpkg -b libldap2" (this will create package libldap2.deb)
7) "sudo dpkg -i libldap2.deb"
Done. Now package manager won't complain about missing dependency anymore.
I really think that there should be easier way to do that (some kind of "package aliases" in synaptic, or anywhere else), but it looks like there isn't. Or it could be avoided if downloaded packages could be edited by hand. But alas, this is the only way to fix dependency.