The plugins completion test was checking completions for the abi
plugins. But the available abi plugins will depend on which
[targets](https://github.com/llvm/llvm-project/blob/42d2ae1034b287eb60563c370dbf52c59b66db20/lldb/source/Plugins/ABI/CMakeLists.txt#L7)
are enabled in the cmake build configuration.

This PR updates the test to check for the json object file instead which
should be enabled on all builds.

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
        ...
```

In #134418 we added support to list/enable/disable `SystemRuntime` and
`InstrumentationRuntime` plugins. We limited it to those two plugin
types to flesh out the idea with a smaller change.

This PR adds support for the remaining plugin types. We now support all
the plugins that can be registered directly with the plugin manager.
Plugins that are added by loading shared objects are still not
supported.