Make modules.dot target more robust.

Only include source-controlled files under src.

Makefile

@@ -1,6 +1,7 @@
 version?=$(shell grep '^[Vv]ersion:' pandoc.cabal | awk '{print $$2;}')
 pandoc=$(shell find dist -name pandoc -type f -exec ls -t {} \; | head -1)
 SOURCEFILES?=$(shell git ls-tree -r master --name-only | grep "\.hs$$")
+PANDOCSOURCEFILES?=$(shell git ls-tree -r master --name-only src | grep "\.hs$$")
 BRANCH?=master
 ARCH=$(shell uname -m)
 DOCKERIMAGE=registry.gitlab.b-data.ch/ghc/ghc4pandoc:9.2.3
@@ -162,10 +163,10 @@ update-website: ## update website and upload
 	make -C $(WEBSITE) upload
 .PHONY: update-website
 
-modules.dot: $(SOURCEFILES)
+modules.dot: $(PANDOCSOURCEFILES)
 	@echo "digraph G {" > $@
 	@echo "overlap=\"scale\"" >> $@
-	@rg '^import.*Text\.Pandoc\.' --with-filename src \
+	@rg '^import.*Text\.Pandoc\.' --with-filename $^ \
 		| rg -v 'Text\.Pandoc\.(Definition|Builder|Walk|Generic)' \
 		| sort \
 		| uniq \