providers: Disable if g:loaded_* exists.

The exists('g:loaded_foo') convention is rather common, and may be
relied upon in some cases. It's also very unlikely that a user or plugin
has any reason to set g:loaded_foo to zero, so the principle of least
surprise can be brushed aside here.

https://github.com/neovim/neovim/issues/6107#issuecomment-279532143

runtime/autoload/health/provider.vim

@@ -226,12 +226,12 @@ function! s:check_python(version) abort
   let pyenv_root = exists('$PYENV_ROOT') ? resolve($PYENV_ROOT) : 'n'
   let venv = exists('$VIRTUAL_ENV') ? resolve($VIRTUAL_ENV) : ''
   let host_prog_var = pyname.'_host_prog'
-  let loaded_var = 'loaded_'.pyname.'_provider'
+  let loaded_var = 'g:loaded_'.pyname.'_provider'
   let python_bin = ''
   let python_multiple = []
 
-  if get(g:, loaded_var, 0) && !exists('*provider#'.pyname.'#Call')
-    call health#report_info('Disabled. g:'.loaded_var.'='.get(g:, loaded_var, 0))
+  if exists(loaded_var) && !exists('*provider#'.pyname.'#Call')
+    call health#report_info('Disabled. '.loaded_var.'='.eval(loaded_var))
     return
   endif
 
@@ -415,9 +415,9 @@ endfunction
 function! s:check_ruby() abort
   call health#report_start('Ruby provider')
 
-  let loaded_var = 'loaded_ruby_provider'
-  if get(g:, loaded_var, 0) && !exists('*provider#ruby#Call')
-    call health#report_info('Disabled. g:'.loaded_var.'='.get(g:, loaded_var, 0))
+  let loaded_var = 'g:loaded_ruby_provider'
+  if exists(loaded_var) && !exists('*provider#ruby#Call')
+    call health#report_info('Disabled. '.loaded_var.'='.eval(loaded_var))
     return
   endif
 

runtime/autoload/provider/python.vim

@@ -4,7 +4,7 @@
 " Associating the plugin with the Python host is the first step because plugins
 " will be passed as command-line arguments
 
-if get(g:, 'loaded_python_provider', 0)
+if exists('g:loaded_python_provider')
   finish
 endif
 let g:loaded_python_provider = 1

runtime/autoload/provider/python3.vim

@@ -4,7 +4,7 @@
 " Associating the plugin with the Python3 host is the first step because
 " plugins will be passed as command-line arguments
 
-if get(g:, 'loaded_python3_provider', 0)
+if exists('g:loaded_python3_provider')
   finish
 endif
 let g:loaded_python3_provider = 1

runtime/autoload/provider/ruby.vim

@@ -1,5 +1,5 @@
 " The Ruby provider helper
-if get(g:, 'loaded_ruby_provider', 0)
+if exists('g:loaded_ruby_provider')
   finish
 endif
 let g:loaded_ruby_provider = 1