[Haskell-beginners] How to build a package?

[John M. Dlugosz]

I used cabal unpack to get the source of a package to read.  Now I want to make a change. 
  How do I tell Haskell Platform to make that (the same way cabal knows how to) and do so 
with the provided files rather than downloading into temp.
I got a package to work before using Leksah, but I'd like to do it directly since the 
cabal file is present and works just fine.

A related issue: what should I do about the local copy of a module name that is also 
installed?  Can I make my project use that instead, without having to globally change the 
name in all the files?