[slachiewicz]

Adds overWriteFiles (user property mdep.unpack.overWriteFiles, default true) to unpack, unpack-dependencies and each artifactItem. With false, an entry whose target file already exists, after the fileMappers are applied, is skipped whatever the timestamps, so an unpack after process-resources keeps the filtered resources and only adds missing files. The existing overWrite* options still decide per artifact whether to unpack at all. Single-file archives (gzip, xz, snappy, zstd) are not affected.

Fixes #917

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