Split write_splicing_manifest and write_config_file. (#2225)

This commit creates the "generate" prefix for each of these functions,
which generates the content of these files in a way that doesn't touch
the ctx variable. This allows us to use these functions in module
extensions, which is required for
#1910.

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Co-authored-by: UebelAndre <[email protected]>

Author: matts1