fix(developer): projects 2.0 internal path enumeration

Restricts enumeration of files for the project to the project folder and
the SourcePath folder. This prevents problems where a project may be in
a folder with many subfolders which would take a long time to enumerate,
and avoids confusion where there are source-type files in other folders.

At the same time, sorts out forward slash vs backslash in paths. While
forward slash works in many scenarios, there are several filename
manipulation functions, such as ExpandFileName, which would build valid
but non-optimal paths when forward slashes were encountered, which
cascaded into files appearing to be different and presentation issues.