indexer: use mmap for writing

Some OSs cannot keep their ideas about file content straight when mixing
standard IO with file mapping. As we use mmap for reading from the
packfile, let's make writing to the pack file use mmap.

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This does remove support for a `NO_MMAP` build to perform indexing, but there isn't much to tell us that anybody actually uses said build for anything. We can add writing emulation if anybody actually needs it.