First the good news.
I downloaded fpc-source -2.4.4_2.4.4._all.deb from Debian and installed it with dpkg and I am now the proud owner of the folder
/usr.share/fpcsrc.
When I start Lazarus, the error about not finding the fpcsrc has gone, and so it looks as if fpcsrc is installed. In fact, I checked in Synaptic and found the check box for fpcsrc was green - it's installed.
Now the bad news.
When I start Lazarus and make a new application, save and run, I get the message
Fatal. Can't find interfaces used by project1