doc: use `is_fictive` instead of relying on the module name

Signed-off-by: Jean Privat <jean@pryen.org>
model: promote `is_fictive` from doc_model

Signed-off-by: Jean Privat <jean@pryen.org>
modelbuilder: automatic mainmodule from run_global_phases is fictive

Signed-off-by: Jean Privat <jean@pryen.org>
nitdoc: replace NitdocContext by ToolContext

Signed-off-by: Jean Privat <jean@pryen.org>
nitdoc: use global phase (process_mainmodule)