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

doc/advopt.txt

@@ -67,6 +67,13 @@ Advanced options:
   --clib:LIBNAME            link an additional C library
                             (you should omit platform-specific extensions)
   --project                 document the whole project (doc2)
+  --docRoot:path            nim doc --docRoot:/foo --project --outdir:docs /foo/sub/main.nim
+                            generates: docs/sub/main.html
+                            if path == @pkg, will use nimble file enclosing dir
+                            if path == @path, will use first matching dir in --path
+                            if path == @default (the default and most useful), will use
+                            best match among @pkg,@path.
+                            if these are nonexistant, will use project path
   --docSeeSrcUrl:url        activate 'see source' for doc and doc2 commands
                             (see doc.item.seesrc in config/nimdoc.cfg)
   --docInternal             also generate documentation for non-exported symbols