More safely resolve 'jest-config'. (#853)

* fix: the jest-config package isn't resolved in a non-flat node_modules

* style: rename jestConfig to jestConfigPkg

* style: refactors and style changes from PR feedback

* fix: remove yarn.lock and updat package-lock.json

* fix: make getJestConfigPkg generic to avoid pulling in jest-config types

* style: use granular imports

Author: iclanton