Dlilley.bugfix improve navigation

[dklilley]

These changes address two issues:

1. "Go to Definition" does not seem to work when using package folders MATLAB-extension-for-vscode#5 - Improves navigation to files within packages when those packages are not on the MATLAB path
2. Definition jumps to local functions that should not be able to be referenced MATLAB-extension-for-vscode#31 - Prevents navigation to private/local functions from other files

For (1), we first try to resolve an identifier via the MATLAB path (as before). But now, if that resolution fails, we temporarily cd into the source file's directory, thereby forcing it onto the path. We then attempt a name resolution again before cding back to the original directory. If the name resolution still fails, we fall back to the code data cache.

For (2), a check was added to ensure that 'private' functions in other files cannot be matched when searching for a definition.