Fix bug: debugger can show base environment on launch

This bug occurs when no function on the callstack has source references;
the easiest way to repro is to call debug(foo::bar); foo::bar(), where
foo is a package for which source references are not available, and bar
is a function in that package.

The problem is that RStudio walks the callstack looking for a function
containing source references, and when it reaches the end and still
hasn't found one, it returns the last function it examined.

The fix is to detect when we're in this case (no sourcerefs anywhere),
and to:
a) debug in the context of the top function on the stack, and
b) show internal functions in the UI (since there are no other kind on
the stack)
Merge pull request #68 from rstudio/bugfix/debug-base

Fix bug: debugger can show base environment on launch