Since GitHub organizes code modules alphabetically and thinks it wise to decorate folder and file items with their last commit message, the structure of the source code is invisible. In particular, the code entry point is not obvious. Even the viewing of file content is constrained by an artificial narrowing of the view width. Just frustrating. (for some reason my version of fsc.exe seems to be searching for FSharp.Core 4.0 in the GAC and failing)
Is this a question/issue for the f# compiler? Is this a comment about ... github?
It is an issue with FSharp source accessibility within the failings of GitHub. It needs to compensate in some way for this failing. For instance, the Fsc project refers to "fscmain.fs", however, such a file was not found by a GitHub repository search. GitHub is not exposing the code intuitively, though it is making it available.
I find the file immediately using the GitHub find file feature

I wasn't as lucky. I was surprise though, seeing as how it is clearly stated in the project file. Unfortunately, I don't have any specifics for restructuring at the moment. Perhaps additional directories...
however, such a file was not found by a GitHub repository search
GitHub search is, unfortunately, not very "smart" right now. @brodyberg's suggestion about using the file finder for search is your best bet for things like this. If you want to understand the topographical organization of the codebase, your best bet is to clone and open in VS.
Structure visibility and accessibility might be good hypertext based project. :)
Please use the template in ISSUE_TEMPLATE.md when reporting bugs. I'll close this since it doesn't appear to fit that format.
Most helpful comment
https://fsharp.github.io/2015/09/29/fsharp-compiler-guide.html