It's something of a moot point but I thought that Source and Goldsource were forked a while back and do share a little bit... it's immaterial anyway.
Well yes, I guess you're right, in the beginning Source was a fork of Goldsource, but as time went by more and more of Source was rewritten so today I would guess there is very little left of Goldsource in Source, so little left you can consider Source a completely new engine...
Source distantly originates from the GoldSrc engine, itself a heavily modified version of John D. Carmack's original Quake engine, as is explained by Valve employee Erik Johnson on the Valve Developer Community:
When we were getting very close to releasing Half-Life (less than a week or so), we found there were already some projects that we needed to start working on, but we couldn't risk checking in code to the shipping version of the game. At that point we forked off the code in VSS to be both $/Goldsrc and /$Src. Over the next few years, we used these terms internally as "Goldsource" and "Source". At least initially, the Goldsrc branch of code referred to the codebase that was currently released, and Src referred to the next set of more risky technology that we were working on. When it came down to show Half-Life 2 for the first time at E3, it was part of our internal communication to refer to the "Source" engine vs. the "Goldsource" engine, and the name stuck.
Source was developed part-by-part from this fork onwards, slowly replacing GoldSrc in Valve's internal projects and explaining in part the reasons behind its unusually modular nature. Valve's development of Source since has been mixture of licensed middleware (Havok Physics, albeit heavily modified, and MP3 playback) and in-house-developed code.
John Carmack commented on his blog in 2004 that "there are still bits of early Quake code in Half-Life 2" and numerous development interfaces are similar to Quake's, at least at a high level - but with both easily likely of being Valve avoiding reinventing the wheel, neither are indicative of a notable reliance on the older engine. Indeed given the fact that Source's 2003 code leak did not produce any such claims it can only be assumed that no incriminating evidence was to be found.
Source distances itself further from Quake as time goes on.