system + python 2

Nessuno ha ancora preso questa issue.

```bash
~
➜ asdf install python 2.7.18
python-build 2.7.18 /home/sergey/.asdf/installs/python/2.7.18
Downloading Python-2.7.18.tar.xz...
-> https://www.python.org/ftp/python/2.7.18/Python-2.7.18.tar.xz
Installing Python-2.7.18...
patching file configure
patching file configure.ac
patching file setup.py
patching file Mac/Tools/pythonw.c
patching file setup.py
patching file Doc/library/ctypes.rst
patching file Lib/test/test_str.py
patching file Lib/test/test_unicode.py
patching file Modules/_ctypes/_ctypes.c
patching file Modules/_ctypes/callproc.c
patching file Modules/_ctypes/ctypes.h
patching file Modules/_ctypes/callproc.c
patching file setup.py
patching file Mac/Modules/qt/setup.py
patching file setup.py
Installed Python-2.7.18 to /home/sergey/.asdf/installs/python/2.7.18

Installing requests[socks] python package... SUCCESS

Installing cchardet python package... SUCCESS

~ took 1m15s
➜ asdf global python system 2.7.18

~
➜ python2
No python2 executable found for python system

~
➜ whereis python2
python2: /home/sergey/.asdf/shims/python2

~
```

Riproduci la sequenza riportata con `asdf install python 2.7.18`, `asdf global python system 2.7.18` e `python2`. Inizia tracciando come il plugin gestisce la versione di sistema e lo shim `python2`. Il lavoro è completo quando l'interprete di sistema selezionato viene risolto in modo coerente, oppure quando viene documentata la limitazione rimanente.

Scritto dal modello di indicizzazione a partire dal testo della issue.