Properly parse packer output when `PACKER_LOG` set to `1`

[shard77]

When PACKER_LOG is set to 1, the output won't be always in machine-readable format, even if specified so in the command.
Hence, parsing is more difficult and will need additional conditionals for text-based matching.

For performance reasons, we can already determine that we should check if PACKER_LOG is enabled.
If so, we would parse differently than if it is disabled.