start.zig: unconditionally import the root source file

stage1 did this by accident by unconditionally semantically analyzing
std.builtin.panic, which imports the root source file to look for a
panic handler override. But stage2 is smarter; it will only semantically
analyze std.builtin.panic if any panic calls are emitted. So for very
simple compilations, the root source file was being ignored, even if it
contained `export` functions in it.