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In-memory representations of the two types of graphs were made
serializable/deserializable to and from JSON. A new module
`serialization` has been created to help with this process.

The end result is one .txt file containing the JSON dump of both the
class and package dependency graphs - more rationale on this format is
included in the docstring of serialization functions.

The performance of the de/serialization is very acceptable (within
a second on my machine). The filesize of the output JSON file is ~7MB,
which I also find acceptable. There are rooms for optimization in the
JSON structure if the filesize becomes an issue in the future.

Bug: 1081889
Change-Id: I03061372e0ed94aed62de58f41fb3f363f543b3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2219817
Commit-Queue: James Long <yjlong@google.com>
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773357}