impl: add support to complete packages

closes #1867

autoload/go/impl.vim

@@ -101,13 +101,29 @@ function! s:root_dirs() abort
   return dirs
 endfunction
 
-function! s:go_packages(dirs) abort
+function! s:go_packages(dirs, arglead) abort
   let pkgs = []
-  for d in a:dirs
-    let pkg_root = expand(d . '/pkg/' . go#util#osarch())
-    call extend(pkgs, split(globpath(pkg_root, '**/*.a', 1), "\n"))
+  for dir in a:dirs
+      " this may expand to multiple lines
+      let scr_root = expand(dir . '/src/')
+      for pkg in split(globpath(scr_root, a:arglead.'*'), "\n")
+          if isdirectory(pkg)
+              let pkg .= '/'
+          elseif pkg !~ '\.a$'
+              continue
+          endif
+
+          " without this the result can have duplicates in form of
+          " 'encoding/json' and '/encoding/json/'
+          let pkg = go#util#StripPathSep(pkg)
+
+          " remove the scr root and keep the package in tact
+          let pkg = substitute(pkg, scr_root, "", "")
+          call add(pkgs, pkg)
+      endfor
   endfor
-  return map(pkgs, "fnamemodify(v:val, ':t:r')")
+
+  return pkgs
 endfunction
 
 function! s:interface_list(pkg) abort
@@ -124,13 +140,24 @@ endfunction
 " Complete package and interface for {interface}
 function! go#impl#Complete(arglead, cmdline, cursorpos) abort
   let words = split(a:cmdline, '\s\+', 1)
+
   if words[-1] ==# ''
-    return s:uniq(sort(s:go_packages(s:root_dirs())))
-  elseif words[-1] =~# '^\h\w*$'
-    return s:uniq(sort(filter(s:go_packages(s:root_dirs()), 'stridx(v:val, words[-1]) == 0')))
-  elseif words[-1] =~# '^\h\w*\.\%(\h\w*\)\=$'
-    let [pkg, interface] = split(words[-1], '\.', 1)
-    echomsg pkg
+    " if no words are given, just start completing the first package we found
+    return s:uniq(sort(s:go_packages(s:root_dirs(), a:arglead)))
+  elseif words[-1] =~# '^\(\h\w.*\.\%(\h\w*\)\=$\)\@!\S*$'
+    " start matching go packages. It's negate match of the below match
+    return s:uniq(sort(s:go_packages(s:root_dirs(), a:arglead)))
+  elseif words[-1] =~# '^\h\w.*\.\%(\h\w*\)\=$'
+    " match the following, anything that could indicate an interface candiate
+    " 
+    "  io.
+    "  io.Wr
+    "  github.com/fatih/color.
+    "  github.com/fatih/color.U
+    "  github.com/fatih/color.Un
+    let splitted = split(words[-1], '\.', 1)
+    let pkg = join(splitted[:-2], '.')
+    let interface = splitted[-1]
     return s:uniq(sort(filter(s:interface_list(pkg), 'v:val =~? words[-1]')))
   else
     return []