Calls to `NewProjectWorkspace` will panic if there is no Pulumi.yaml found in the folder hierarchy. Simple repo:
```
git init
pulumi init
pulumi stack init x
```
`DetectPackage` will search until it gets to the `.pulumi` directory or finds a `Pulumi.yaml` file. In the case of the former, we pass "" to `LoadPackage` which then asserts because the file doesn't exist.
We now detect this condition and surface an error to the user. The error text is patterned after running a git command when there is no .git folder found:
fatal: Not a git repository (or any of the parent directories): .git
bd5e54d63e