refactor!: remove nvim and provider module for checkhealth

The namespacing for healthchecks for neovim modules is inconsistent and
confusing. The completion for `:checkhealth` with `--clean` gives

```
nvim
provider.clipboard
provider.node
provider.perl
provider.python
provider.ruby
vim.lsp
vim.treesitter
```

There are now three top-level module names for nvim: `nvim`, `provider`
and `vim` with no signs of stopping. The `nvim` name is especially
confusing as it does not contain all neovim checkhealths, which makes it
almost a decoy healthcheck.

The confusion only worsens if you add plugins to the mix:

```
lazy
mason
nvim
nvim-treesitter
provider.clipboard
provider.node
provider.perl
provider.python
provider.ruby
telescope
vim.lsp
vim.treesitter
```

Another problem with the current approach is that it's not easy to run
nvim-only healthchecks since they don't share the same namespace. The
current approach would be to run `:che nvim vim.* provider.*` and would
also require the user to know these are the neovim modules.

Instead, use this alternative structure:

```
vim.health
vim.lsp
vim.provider.clipboard
vim.provider.node
vim.provider.perl
vim.provider.python
vim.provider.ruby
vim.treesitter
```

and

```
lazy
mason
nvim-treesitter
telescope
vim.health
vim.lsp
vim.provider.clipboard
vim.provider.node
vim.provider.perl
vim.provider.python
vim.provider.ruby
vim.treesitter
```

Now, the entries are properly sorted and running nvim-only healthchecks
requires running only `:che vim.*`.

test/functional/plugin/health_spec.lua

@@ -5,7 +5,7 @@ local Screen = require('test.functional.ui.screen')
 local clear = n.clear
 local curbuf_contents = n.curbuf_contents
 local command = n.command
-local eq, neq, matches = t.eq, t.neq, t.matches
+local eq, matches = t.eq, t.matches
 local getcompletion = n.fn.getcompletion
 local insert = n.insert
 local exec_lua = n.exec_lua
@@ -36,16 +36,15 @@ describe(':checkhealth', function()
     clear()
     -- Do this after startup, otherwise it just breaks $VIMRUNTIME.
     command("let $VIM='zub'")
-    command('checkhealth nvim')
+    command('checkhealth vim.health')
     matches('ERROR $VIM .* zub', curbuf_contents())
   end)
 
   it('completions can be listed via getcompletion()', function()
     clear()
-    eq('nvim', getcompletion('nvim', 'checkhealth')[1])
-    eq('provider.clipboard', getcompletion('prov', 'checkhealth')[1])
+    eq('vim.health', getcompletion('vim', 'checkhealth')[1])
+    eq('vim.provider.clipboard', getcompletion('vim.prov', 'checkhealth')[1])
     eq('vim.lsp', getcompletion('vim.ls', 'checkhealth')[1])
-    neq('vim', getcompletion('^vim', 'checkhealth')[1]) -- should not complete vim.health
   end)
 
   it('completion checks for vim.health._complete() return type #28456', function()