cubicweb #2841188 site_cubicweb files should be loaded before schema deserialization [validation pending]

as they may contain information necessary to understand the schema once we can add new base types (see #XXX)

prioritynormal
typeenhancement
done in3.17.0
load1.500
load left0.000
closed by#3bbd416b09ec [repo] straightforward bootstrap sequence. Closes #2841188
patch[repo] straightforward bootstrap sequence. Closes #2841188 [applied]