I found it, the tricky part was there are so many repos just listed there on their git repo page (http://git.eclipse.org/c/), you kind of have to inspect each page, it is listed under the category "platform", as opposed to all the plugin ID's being listed in alphabetic order:
For org.eclipse.debug.core (where IProcess is), it is here:
http://git.eclipse.org/c/platform/eclipse.platform.debug.git/tree/org.eclipse.debug.core
Answer from Zombies on Stack OverflowI found it, the tricky part was there are so many repos just listed there on their git repo page (http://git.eclipse.org/c/), you kind of have to inspect each page, it is listed under the category "platform", as opposed to all the plugin ID's being listed in alphabetic order:
For org.eclipse.debug.core (where IProcess is), it is here:
http://git.eclipse.org/c/platform/eclipse.platform.debug.git/tree/org.eclipse.debug.core
Depending on your needs, The Eclipse Common Build Infrastructure (CBI) might help.
The CBI project has converted Eclipse into a Tycho project which means you can get all the sources and compile them into a working Eclipse build with 2-3 commands.