Dart/Flutter LSP configuration options are not respected
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I'm using Neovim with it's build-in LSP.
So it seems like an old option `[layers.override_cmd]` is not respected for my configuration.
This causes troubles while trying to replace Dart SDK with Flutter SDK. Especially the dart binary itself.

SpaceVim calls system's Dart binary which version (probably) differs from a Dart binary version in Flutter SDK in my case.
Which consequently causes an error:
```
[ERROR][2022-05-22 15:00:20] .../vim/lsp/rpc.lua:420    "rpc"   "dart"  "stderr"        "Wrong full snapshot version, expected '1441d6b13b8623fa7fbf61433abebd31' found 'd56742caf7b3b3f4bd2df93a9bbb5503'\n"
```
Probably SpaceVim tries to call LSP server this way:
```
dart /opt/flutter/bin/cache/dart-sdk/bin/snapshots/analysis_server.dart.snapshot --lsp
```
In this case this is the wrong "dart":
```
$ whereis dart
dart: /usr/bin/dart /opt/dart-sdk/bin/dart /opt/flutter/bin/dart
```
because `/usr/bin/dart` is a symlink to `/opt/dart-sdk/bin/dart`

## Expected behavior, english is required
LSP should be called this way
```
/opt/flutter/bin/cache/dart-sdk/bin/dart /opt/flutter/bin/cache/dart-sdk/bin/snapshots/analysis_server.dart.snapshot --lsp
```

## The reproduce ways from Vim starting (Required!)
Just open any dart file inside of a flutter project

## Config
```
[[layers]]
    name = "lsp"
    enabled_clients = [
        'dartls',
        ...
    ]
    filetypes = [
        "dart",
        ...
    ]
    [layers.override_cmd]
      dart = [
          "/opt/flutter/bin/cache/dart-sdk/bin/dart",
          "/opt/flutter/bin/cache/dart-sdk/bin/snapshots/analysis_server.dart.snapshot",
          "--lsp"
      ]
      ...

[[layers]]
    name = "lang#dart"
    dart_sdk_path = "/opt/flutter/bin/cache/dart-sdk/"
```

As You see I've also found `dart_sdk_path` option in the docs but it also does nothing.
Looks like my config is now a mixture of an old "classic" LSP config and vim's built-in LSP config. But how to force built-in LSP to use custom paths? BTW the question is actual for any LSP+language.

Though... If I completely remove the `override_cmd` section - SpaceVim blaims it cannot execute LSP. So it seems like it embraces the config but just does not respect it's first argument (path to dart binary).

Obviously removing dart from the system and adding `/opt/flutter/bin` to `$PATH` helped as a workaround.

## Output of the `:SPDebugInfo!`
[debug-info.txt](https://github.com/SpaceVim/SpaceVim/files/8750075/debug-info.txt)

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