Better install --dry-run

Key point here is that the 'cleanup' step is omitted.

Formatting of data is also improved.

We may wish to make the cleanup step less agressive once an uninstall
target is available: one might remove only the files matching those to
be created. (Or is that a bad idea? Aligns with what uninstall should
do in the first place.)

l3build.lua

@@ -2201,22 +2201,26 @@ function install()
   set_program_name("latex")
   local texmfhome = options["texmfhome"] or var_value("TEXMFHOME")
   local installdir = texmfhome .. "/tex/" .. moduledir
-  errorlevel = cleandir(installdir)
-  if errorlevel ~= 0 then
-    return errorlevel
-  end
-  print()
-  for _,i in ipairs(installfiles) do
-    if options["dry-run"] then
-      for _,file in pairs(filelist(unpackdir,i)) do
-        print("Would install " .. file)
+  if options["dry-run"] then
+    print("\n" .. "Installation root: " .. installdir
+      .. "\n" .. "Intallation files:"
+    )
+    for _,filetype in ipairs(installfiles) do
+      for _,file in pairs(filelist(unpackdir,filetype)) do
+        print("- " .. file)
       end
-    else
-      errorlevel = cp(i, unpackdir, installdir)
     end
+  else
+    errorlevel = cleandir(installdir)
     if errorlevel ~= 0 then
       return errorlevel
     end
+    for _,filetype in ipairs(installfiles) do
+      errorlevel = cp(filetype, unpackdir, installdir)
+      if errorlevel ~= 0 then
+        return errorlevel
+      end
+    end
   end
   return 0
 end