Build Errors in Package.swift are not reported in problems

[adam-fowler]

Describe the bug
An error in the Package.swift that is not a syntax error that occurs at build time is not reported in the Problems view

To Reproduce
Steps to reproduce the behavior:

1. Create new project
2. Add a dependency to the target in the project to a module that does not exist
3. Build (Shift+Cmd+B)
4. No errors are reported in the problems view but the project failed to build

Expected behavior
Errors are reported in the Problems view

[award999]

Looked at this. Would need some work from SwiftPM to get the location for the diagnostic