luci-lua-runtime: also search luci.dispatcher scope when looking up values

Some legacy templates access dispatcher functions such as `build_url()`
without explicit scope, which worked in the old runtime since the template
view namespace was instantiated from the dispatcher.

Ensure that this continues to function in the emulated runtime.

Fixes: #6082
Signed-off-by: Jo-Philipp Wich <jo@mein.io>