Fix bootstrap plugin fallback guards

[fallintoplace]

Summary

• Use the matching *_PLUGIN_provided option when deciding whether PMI2, PMI, and UID bootstrap paths may fall back to legacy plugin names.
• Keeps explicit plugin configuration from being overridden by fallback decisions from another bootstrap type.

Testing

• git diff --check
• CMake configure attempted with tests enabled and unrelated build pieces disabled; blocked locally because CUDA toolkit / nvcc is not installed (Failed to find nvcc).