refactor(vim.version): cleanup

- version.cmp(): assert valid version
- add test for loading vim.version (the other tests use shared.lua in
  the test runner)
- reduce test scopes, reword test descriptions

runtime/lua/vim/_init_packages.lua

@@ -51,7 +51,10 @@ end
 -- builtin functions which always should be available
 require('vim.shared')
 
-vim._submodules = { inspect = true }
+vim._submodules = {
+  inspect = true,
+  version = true,
+}
 
 -- These are for loading runtime modules in the vim namespace lazily.
 setmetatable(vim, {
@@ -69,9 +72,6 @@ setmetatable(vim, {
         t[key] = val
         return t[key]
       end
-    elseif key == 'version' then
-      t[key] = require('vim.version')
-      return t[key]
     end
   end,
 })