Add internal link checker

This should make the link rewriting process easier. As this does
the same job as epubChecker, it can probably be removed once
epubChecker is integrated into the build process.

Makefile

@@ -59,6 +59,7 @@ out/plfa.epub: out/ $(AGDA) $(LUA) epub.css fonts/*.ttf
 		--css=epub.css \
 		--epub-embed-font='fonts/*.ttf' \
 		--lua-filter include-files.lua \
+		--lua-filter check-internal-links.lua \
 		--lua-filter default-code-class.lua -M default-code-class=agda \
 		--standalone \
 		--toc --toc-depth=2 \

check-internal-links.lua

@@ -0,0 +1,27 @@
+-- Source: https://github.com/jgm/pandoc/issues/1621#issuecomment-613336520
+
+local identifiers = {}
+
+function Block (b)
+  if b.identifier then
+    identifiers[b.identifier] = true
+  end
+end
+
+function Inline (i)
+  if i.identifier then
+    identifiers[i.identifier] = true
+  end
+end
+
+function Link (l)
+  local anchor = l.target:match('#(.*)')
+  if anchor and not identifiers[anchor] then
+    io.stderr:write("broken link: " .. anchor .. "\n")
+  end
+end
+
+return {
+  {Block = Block, Inline = Inline},
+  {Link = Link}
+}