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

compiler/commands.nim

@@ -413,6 +413,8 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
   of "docseesrcurl":
     expectArg(conf, switch, arg, pass, info)
     conf.docSeeSrcUrl = arg
+  of "docroot":
+    conf.docRoot = if arg.len == 0: "@default" else: arg
   of "mainmodule", "m":
     discard "allow for backwards compatibility, but don't do anything"
   of "define", "d":