idl: Fix potential panic on external type resolution

[acheroncrypto]

Problem

find_path function used in non-Anchor external type resolution can potentially panic if the file we're looking for doesn't exist in any of the ancestors.

This can happen in rare cases such as the recursive external type resolution described in https://github.com/coral-xyz/anchor/pull/2946.

Summary of changes

Rewrite the find_path function without a potential panic.

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