Fix file paths in generated source jars

[alexenvoy]

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When using the Bazel zipper to generate the source jar, the entire path starting from bazel-out is sent to the ijar tool, which becomes the paths to the source files inside the generated jar. To remove this unnecessary prefix and produce source jars with directory structure matching standard Java source jars, we can use the dest=src argument style for the ijar tool, using each file's relative paths as the dest part. This produces source jars with the expected directory structure.