PackageFileHeapWriter: Small style fix
PackageWriterImpl: Fix reading the package-info in update mode
PackageFileHeapAccessorBase: small refactoring

* Pull method DecompressChunkData() out of ReadAndDecompressChunkData()
  for potential reuse.
* Also fix/improve some error output strings.
PackageFileHeapWriter: Fix update case

* Pull _UnwriteLastPartialChunk() out of Reinit() for reuse.
* _UnwriteLastPartialChunk(): fPendingDataSize wasn't set.
* _PushChunks(): Some simplifications for clarity.
* ChunkBuffer/RemoveDataRanges(): Use data reading and decompression
  methods provided by our base class instead of duplicating the
  implementation.
* RemoveDataRanges():
  - _FlushPendingData() before starting, so we don't ignore the pending
    data and _UnwriteLastPartialChunk() when done, so a partial chunk
    is read back into the pending data buffer.
  - fUncompressedHeapSize wasn't reset before the main processing loop,
    thus resulting in an erroneous size later on.