Revisit how to set path of project being added/upgraded

@ecraig12345 2019-02-21 01:21:21发表于 microsoft/just Needs: Design :thought_balloon:

Currently, the way to set the project being operated on by addPackageTask or upgradePackageTask is to set projectPath in just-scripts-utils/paths. It would be clearer to pass the project path as an optional parameter to a task factory instead. (The default would still be process.cwd().)