Is the hgweb.cgi script only in the source package or is it provided with the PPA?
If it's only in the source, do I have to take any other files for it to work, or just hgweb.cgi in the top level folder?
I'm not using any PPA, but the standard package has it in /usr/share/doc/mercurial-common/examples/hgweb.cgi. There are a few alternatives as well (WSGI, etc), that all use a common backend.
If anyone else is wondering, you need hgweb.cgi (either from the location in Tobu's answer) or from the source. You'll also need the template files and static file (contains logo and other images) from the templates folder (you don't need the hgweb folder, that is just source)
You can find the most up to date version of hgweb.cgi in the mercurial source repository over at http://selenic.com/repo
http://www.selenic.com/repo/hg-stable/raw-file/02aa06a021a0/hgweb.cgi
