* Introduce a RESOURCE_DIR to avoid relative hardcoded paths

- Introduce a RESOURCES_DIR similar to other *_DIR
- Add a resources_directory parameter to lookup_vendor() to avoid possible
  hardcoded directory and - if not provided - pick up the default
  RESOURCES_DIR.

* Avoid possible hardcoded relative paths and use RESOURCE_DIR instead

(Otherwise ssh_keys are tried to picked up in in ${PWD} (and mostly
likely failing))

* Avoid hardcoded relative paths and use MODULES_DIR instead

Commit id 9380c047 (probably
accidentally) removed EXPLOITS_DIR use leading to loading
routersploit/modules/exploits relative to the current directory
instead of the installed ones in MODULES_DIR.

* Avoid hardcoded relative paths and use utils.index_modules() instead

* Avoid hardcoded relative paths and use utils.index_modules() instead

* G/C no longer used import/functions

* G/C no longer used module import

* Reintroduce encode and fix a copypasto

* Add missing `.'

Spotted by @lucyoa, thanks!

* Fix a regression on the semantic/name of payloads

payloads should be listed/selected as `<architecture>/<payload>', not
`payloads.<architecture>.<payload>'.

Thanks to @lucyoa!