[loader] try to load code from deprecated paths

* custom loader can try to load files from new paths
  even when the caller code refers to the old ones
* print deprecation warnings with suggestions how to fix them

gateway/src/apicast/executor.lua

@@ -4,6 +4,8 @@
 -- when calling the policy chain methods. This 'context' contains information
 -- shared among policies.
 
+require('apicast.loader') -- to load code from deprecated paths
+
 local policy_chain = require('apicast.policy_chain')
 local policy = require('apicast.policy')
 local linked_list = require('apicast.linked_list')

gateway/src/apicast/loader.lua

@@ -0,0 +1,58 @@
+--- APIcast source loader
+-- Loading this module will add a new source code loader to package.searchers.
+-- The searcher is going to print deprecation warnings when apicast source is loaded
+-- through old or non prefixed paths.
+-- We can rename files and set up an alias here so we don't break customer's code and
+-- print a deprecation warning.
+
+local loadfile = loadfile
+
+local map = {
+  ['apicast'] = 'apicast.policy.apicast'
+}
+
+local function loader(name, path)
+  local file, err = package.searchpath(name, path)
+
+  if file then
+    file, err = loadfile(file)
+  end
+
+  return file, err
+end
+
+local function prefix_loader(name, path)
+  local prefixed = 'apicast.' .. name
+  local found, err = loader(prefixed, path)
+
+  if found then
+    ngx.log(ngx.STDERR, 'DEPRECATION: when loading apicast code use correct prefix: require("', prefixed, '")')
+  end
+
+  return found or err
+end
+
+local function rename_loader(name, path)
+  local new = map[name]
+  local found, err = loader(new, path)
+
+  if found then
+    ngx.log(ngx.WARN, 'DEPRECATION: file renamed, change: require("', name, '")' ,' to: require("', new, '")')
+  end
+
+  return found or err
+end
+
+local function apicast_namespace(name)
+  local path = package.path
+
+  if not package.searchpath(name, path) then
+    if map[name] then
+      return rename_loader(name, path)
+    else
+      return prefix_loader(name, path)
+    end
+  end
+end
+
+table.insert(package.searchers, apicast_namespace)