Fail if init_module_paths called more than once

MSP-11672

Calling init_module_paths takes 6 seconds on my machine even when there are no
files to that are changed just because it takes that long to walk the
directories and gather the mtime for each file.  Therefore, calling it
more than once should be avoided.  Also, there is no reason to call it
twice as to add paths later, `modules.add_module_paths` should be used.

lib/msf/base/simple/framework/module_paths.rb

@@ -7,26 +7,32 @@ module Msf
         #
         # @return [void]
         def init_module_paths(opts={})
-          # Ensure the module cache is accurate
-          self.modules.refresh_cache_from_database
+          if @module_paths_inited
+            fail "Module paths already initialized.  To add more module paths call `modules.add_module_path`"
+          else
+            # Ensure the module cache is accurate
+            self.modules.refresh_cache_from_database
 
-          add_engine_module_paths(Rails.application, opts)
+            add_engine_module_paths(Rails.application, opts)
 
-          Rails.application.railties.engines.each do |engine|
-            add_engine_module_paths(engine, opts)
-          end
+            Rails.application.railties.engines.each do |engine|
+              add_engine_module_paths(engine, opts)
+            end
 
-          # Initialize the user module search path
-          if (Msf::Config.user_module_directory)
-            self.modules.add_module_path(Msf::Config.user_module_directory, opts)
-          end
+            # Initialize the user module search path
+            if (Msf::Config.user_module_directory)
+              self.modules.add_module_path(Msf::Config.user_module_directory, opts)
+            end
 
-          # If additional module paths have been defined globally, then load them.
-          # They should be separated by semi-colons.
-          if self.datastore['MsfModulePaths']
-            self.datastore['MsfModulePaths'].split(";").each { |path|
-              self.modules.add_module_path(path, opts)
-            }
+            # If additional module paths have been defined globally, then load them.
+            # They should be separated by semi-colons.
+            if self.datastore['MsfModulePaths']
+              self.datastore['MsfModulePaths'].split(";").each { |path|
+                self.modules.add_module_path(path, opts)
+              }
+            end
+
+            @module_paths_inited = true
           end
         end