Change FindDependencies to optionally follow symlinks

[thequailman]

I am using the FindDependencies function in this library to gather jsonnet files for future rendering. The current implementation of getAbsPath follows symlinks, which makes it very hard to determine later on what the file path should be when using a custom Importer. This change adds a flag to getAbsPath to make the symlink resolution optional. jsonnet-deps will still follow symlinks/there should be no behavior change for users.

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