Have you made a contribution to ComponentClassResolver? Does it work without the namespace alias?
Either:
<html
xmlns:t="http://tapestry.apache.org/schema/tapestry_5_3.xsd"
xmlns:p="tapestry:parameter"
t:type="mylib/myLayout"
>
<p:toolbar>...</p:toolbar>
</html>
Or
<t:mylib.mylayout
xmlns:t="http://tapestry.apache.org/schema/tapestry_5_3.xsd"
xmlns:p="tapestry:parameter"
>
<p:toolbar>...</p:toolbar>
</t:mylib.mylayout>
Full documentation on component libraries here.