[SR-6064] Warn when creating a package dependency without a target dependency

[bitjammer]

Previous ID	SR-6064
Radar	None
Original Reporter	@bitjammer
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Package Manager
Labels	Bug
Assignee	None
Priority	Medium

md5: 8dfd6f5a226a3960e04bddf1182d2fee

Issue Description:

When setting up a project or adding a new dependency, you typically add a package dependency in `dependencies`. It's pretty easy to forget to add a target dependency in `targets`. When generating an Xcode project or building, we should warn if you don't use a package dependency and remind the user that they need to select which targets from a package that you need.

[aciidgh]

We should be able to emit this warning by checking that at least one product from each dependency package is being referenced. This can be done during the package graph loader step.

[hartbit]

There is a valid use for a dependency with no targets depending on it: once my PR #1381 is merged, you can depend on a package with an executable product and run it with {{swift run executable}}, which is very useful for tools like SwiftLint, Sourcery, etc...

We have several options:

• Go forward as-is with this bug, which will generate a warning for valid use-cases.

• Go forward with this bug but omit the warning when the dependency contains an executable product.

• Not go forward with this bug.

What do you think @aciidb0mb3r and @bitjammer?

[aciidgh]

I think David's PR can be considered a fix for this bug!