cmd/dist: detect import cycles instead of deadlocking

[heathdutton]

Summary

Previously, if there was an import cycle in the packages being built (e.g., package A imports B and B imports A), the build would deadlock with "all goroutines are asleep - deadlock!" because each package's goroutine would block waiting for the other to complete.

Now we detect import cycles before blocking on dependencies by tracking which packages each goroutine is waiting on and checking for cycles using depth-first search. When a cycle is detected, we report it with a clear message:

• Self-import: import cycle: go/ast -> go/ast
• Indirect cycle: import cycle: go/ast -> go/scanner -> go/token -> go/ast

Test plan

• Tested self-import cycle detection (go/ast imports go/ast)
• Tested indirect import cycle detection (go/token imports go/ast, go/ast imports go/token)
• Verified normal builds complete successfully
• all.bash passes: ALL TESTS PASSED

Fixes #76439

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