Extracted from #14265.

This situation needs to be tested and dealt with:

1. Dependency Foo depends on Dependency Bar, which runs Bar's build.zig.
2. Dependency Bar depends on Dependency Foo, which runs Foo's build.zig.
3. Infinite loop occurs.

This would not be an issue when depending on zig packages without executing build.zig logic (#14282).

Marking as "bug" because I'm pretty sure this will be an infinite loop, when instead it should be a nice error message.

The user is supposed to avoid this problem by putting their call to dependency() inside an if statement to break the loop.