doc: get package from LSP

Get the package name of the identifier under the cursor using the doc
link provided by lsp. This fixes two problems: performance of trying to
rely on go list via go#tool#Imports()  to get the package name and that
the package name is sometimes being set incorrectly to a method
receiver.

Fixes fatih#3546

autoload/go/doc.vim

@@ -203,13 +203,33 @@ endfunction
 function! s:godocWord(...) abort
   let words = a:000
   if a:0 is 0
-    let oldiskeyword = &iskeyword
-    " TODO(bc): include / in iskeyword when filetype is godoc?
-    setlocal iskeyword+=.
-    let word = expand('<cword>')
-    let &iskeyword = oldiskeyword
-    let word = substitute(word, '[^a-zA-Z0-9\\/._~-]', '', 'g')
-    let words = split(word, '\.\ze[^./]\+$')
+    if &filetype isnot 'godoc'
+      " TODO(bc): use LSP textdocumment/documentSymbol to get the symbols for
+      " the current document and figure out if the package of the symbol under
+      " the cursor when a:0 is 0
+      let [l:out, l:err] = go#lsp#DocLink()
+      if l:err
+        call go#util#EchoError(l:out)
+        return []
+      endif
+
+      if len(l:out) == 0
+        call go#util#EchoWarning("fubar")
+        return []
+      endif
+
+      " strip out any version string in the doc link path.
+      let l:out = substitute(l:out, '@v[^/]\+', '', '')
+      let words = split(l:out, '#')
+    else
+      let oldiskeyword = &iskeyword
+      " TODO(bc): include / in iskeyword when filetype is godoc?
+      setlocal iskeyword+=.
+      let word = expand('<cword>')
+      let &iskeyword = oldiskeyword
+      let word = substitute(word, '[^a-zA-Z0-9\\/._~-]', '', 'g')
+      let words = split(word, '\.\ze[^./]\+$')
+    endif
   endif
 
   if !len(words)
@@ -229,14 +249,6 @@ function! s:godocWord(...) abort
     let exported_name = words[1]
   endif
 
-  if &filetype isnot 'godoc'
-    let packages = go#tool#Imports()
-    if has_key(packages, pkg)
-      let pkg = packages[pkg]
-    endif
-  endif
-
-
   return [pkg, exported_name]
 endfunction