Consolidate *MutatorContext and ModuleContext into BaseModuleContext

Following 99bdb2ab4fd5168a71a20aecf10611425be47ec4 in build/blueprint,
move more methods into BaseModuleContext.

This reapplies I9f8df94f1ae2b55d3cccf7b9468247f3e7cb2ebd after fixing
missing errors thrown for missing defaults modules when
AllowMissingDependencies == true.

Test: m checkbuild
Test: defaults_test.go
Change-Id: Ia17b2bcbf2bac6889c419b2e73953946f6aa40ad