neovim: Use local (venv, conda) python3 as neovim host if appropriate

Previously, neovim's host python3 is configured in the order of
system python (/usr/local/bin/python3, /usr/bin/python3) and then
local python3 from $PATH (e.g. virtualenv or anaconda). This means
as long as system python exists, it always becomes neovim's host python.

From now, local python3 is more prioritized given that 'neovim' package
is installed (otherwise, use system python as default host python
so that it doesn't force users to have neovim package installed on it).

This will allow us to use local python3 as a neovim's host python
if we want (by installing 'neovim' package), which could be quite
useful in some use cases such as numirias/semshi#19 .

nvim/init.vim

@@ -14,16 +14,27 @@ if empty(glob(g:python_host_prog))
     let g:python_host_prog = '/usr/bin/python'
 endif
 
-let g:python3_host_prog = '/usr/local/bin/python3'
-if empty(glob(g:python3_host_prog))
-    " Fallback if not exists
-    let g:python3_host_prog = '/usr/bin/python3'
-endif
-if empty(glob(g:python3_host_prog)) && executable("python3")
-    " Fallback to local/venv python3 if not exists
-    let g:python3_host_prog = substitute(system("which python3"), '\n\+$', '', '')
+
+let g:python3_host_prog = ''
+
+if executable("python3")
+    " get local python from $PATH (virtualenv/anaconda or system python)
+    let s:python3_local = substitute(system("which python3"), '\n\+$', '', '')
+    " detect whether neovim package is installed
+    let s:python3_neovim_path = substitute(system("python3 -c 'import neovim; print(neovim.__path__)' 2>/dev/null"), '\n\+$', '', '')
+    if !empty(s:python3_neovim_path)
+        " neovim available, use it as a host python3
+        let g:python3_host_prog = s:python3_local
+    endif
+else
+    let s:python3_local = ''
 endif
 
+" Fallback to system python3 (if not exists)
+if empty(glob(g:python3_host_prog)) | let g:python3_host_prog = '/usr/local/bin/python3' | endif
+if empty(glob(g:python3_host_prog)) | let g:python3_host_prog = '/usr/bin/python3'       | endif
+if empty(glob(g:python3_host_prog)) | let g:python3_host_prog = s:python3_local          | endif
+
 " VimR support {{{
 " @see https://github.com/qvacua/vimr/wiki#initvim
 if has('gui_vimr')