fix #13150 nim doc --project now works reliably (#13223)

* fix #13150 `nim doc --project` works with duplicate names and with imports below main project file

* add to help; fixup after #13212 isRelativeTo got merged
* fix test tests/compilerapi/tcompilerapi.nim
* remove nimblePkg field; compute on the fly instead
* kochdocs: compiler docs now under compiler/
* --docRoot now has smart default: best among @pkg, @path

nimdoc/testproject/expected/subdir/subdir_b/utils.html

@@ -17,7 +17,7 @@
 <link href='https://fonts.googleapis.com/css?family=Source+Code+Pro:400,500,600' rel='stylesheet' type='text/css'/>
 
 <!-- CSS -->
-<title>utils</title>
+<title>subdir/subdir_b/utils</title>
 <link rel="stylesheet" type="text/css" href="../../nimdoc.out.css">
 
 <script type="text/javascript" src="dochack.js"></script>
@@ -71,7 +71,7 @@ function main() {
 <body onload="main()">
 <div class="document" id="documentId">
   <div class="container">
-    <h1 class="title">utils</h1>
+    <h1 class="title">subdir/subdir_b/utils</h1>
     <div class="row">
   <div class="three columns">
   <div class="theme-switch-wrapper">