android_test.go already sets up a global buildDir, no need to create
one in each test.
Test: All soong tests
Change-Id: Ib85cbf78c604ad3cef64aa00b6bd812a81496e9e
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