set underlaydirbase in docwiki.setup

4291f2e3d7 made
just setting underlaydir not enough to change
where other underlays are loaded from. While generally
a good thing, that broke building of the docwiki
from the source tree, because underlaydirbase
was not pointing there.

Setting underlaydirbase in the setup file is a solution,
albeit slightly hacky (since the variable is undocumented and internal).

docwiki.setup

@@ -6,6 +6,7 @@ use IkiWiki::Setup::Standard {
 	srcdir => "doc",
 	destdir => "html",
 	templatedir => "templates",
+	underlaydirbase => "underlays",
 	underlaydir => "underlays/basewiki",
 	discussion => 0,
 	exclude => qr/\/discussion|bugs\/*|todo\/*/,