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

The Eclipse SDK ships with Source bundles. These bundles contain the source code used for that version, but you can't really build them to get the IDE. These are shipped so that developers can see / use the source while they're developing their own application.

Depending on what you're trying to do in your research project, these bundles might be enough. For example, if you want to do some analysis techniques or code metrics, you can probably just use these. You can download old SDKs from the archive server.

If this is not enough, and you need to re-create the exact source code that was used for each release, you'll need to clone the Git repositories and checkout the tag associated with Each release. This is going to be complicated since there are about 22 different source repos for the IDE alone. Here is a link to the list of repos.