Add gazebodistro downstream library topological ordering

[methylDragon]

From: gazebo-tooling/release-tools#710

Description

This package clones gazebodistro in a temp directory, and uses it to calculate information about the LATEST version of downstream packages, including:

• Generating valid bump phases via topological ordering, so downstream packages will always be merged AFTER upstream ones
• Generating a list of all downstream, (explicit) dependants of any participant libraries

Usage

./gazebodistro_downstream_parser "<lib>'

Alternatively, do it with semicolon delimiters! (Making sure to use quotes)
./gazebodistro_downstream_parser "<lib_1>;<lib_2>;...'

Example Invocation

methyldragon@methyldragon-ThinkPad-T480:~/Desktop/gazebodistro$ ./gazebodistro_downstream_parser.py "ign-rendering;ign-launch"
PARSING TARGETS: ['ign-rendering', 'ign-launch']
Cloning into 'gazebodistro'...
remote: Enumerating objects: 1562, done.
remote: Counting objects: 100% (552/552), done.
remote: Compressing objects: 100% (376/376), done.
remote: Total 1562 (delta 402), reused 235 (delta 170), pack-reused 1010
Receiving objects: 100% (1562/1562), 234.45 KiB | 1.67 MiB/s, done.
Resolving deltas: 100% (998/998), done.

=== EXPLICIT DEPENDANTS ===
All dict values are dependants of their keys!

{'ign-gazebo': ['ign-launch'],
 'ign-gui': ['ign-gazebo', 'ign-launch'],
 'ign-launch': [],
 'ign-rendering': ['ign-gazebo', 'ign-gui', 'ign-launch', 'ign-sensors'],
 'ign-sensors': ['ign-gazebo', 'ign-launch']}

=== TOPOLOGICALLY ORDERED MERGE WAVES ===
(Merge from highest number to lowest number!)

4 ['ign-rendering']
3 ['ign-sensors', 'ign-gui']
2 ['ign-gazebo']
1 ['ign-launch']