cmd/dist: detect import cycles instead of deadlocking

[heathdutton]

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 each dependency by
tracking an "install stack" of which package is waiting on which.
Before waiting for a dependency, we walk the stack to check if that
dependency is already waiting on us. When a cycle is detected, we
report it with a clear message like "import cycle: A -> B -> A".

Tested:

• Self-import cycle detection (go/ast imports go/ast)
• Indirect import cycle detection (go/token imports go/ast creates
  cycle through go/ast -> go/token -> go/ast)
• Normal builds complete successfully
• all.bash passes

Fixes #76439

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