[migrate] Sort output by package and then by version bump.

Restructures the migration summary builder to output the results grouped
by package, then by the versions bumped (e.g., `3.12.0 -> 3.13.0`), and
finally by the steps.

`PackageMigrationSummary` and `VersionMigrationSummary` model the
package and version data.

Change-Id: I2014bb29883c2f55641357d7e8e7f69ae1104e7d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/542460
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>