This commit adds completion support for the plugin commands. It will try
to complete partial namespaces to the full namespace string. If the
completion input is already a full namespace string then it will add all
the matching plugins in that namespace as completions.
This lets the user complete to the namespace first and then tab-complete
to the next level if desired.
```
(lldb) plugin list a<tab>
Available completions:
abi
architecture
(lldb) plugin list ab<tab>
(lldb) plugin list abi<tab>
(lldb) plugin list abi.<tab>
Available completions:
abi.SysV-arm64
abi.ABIMacOSX_arm64
abi.SysV-arm
...
```
fccae859bc