Handle assignments to global variables from the init function

[lizard-boy]

Original PR: uber-go/nilaway#253

[greptile-apps]

PR Summary

This PR adds new detection logic for assignments to global variables from init functions, enhancing how NilAway initializes and processes global variables.

• In /assertion/global/analyzer.go, getInitFuncDecls now collects init functions with a visited set to avoid infinite recursion; duplicate entries may occur across init functions.
• In /assertion/global/globalvarinit.go, analyzeValueSpec and getGlobalConsumers now accept init declarations, and hasGlobalVarAssignInInitFunc is added to detect global assignments.
• Consider caching repeated calls to hasGlobalVarAssignInInitFunc to improve efficiency and ensure proper logging using logger.error for failures.

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