un-special-case startup code in the std lib

Previously, the compiler had special logic to determine whether to
include the startup code, which was in `std/special/start.zig`. Now,
the file is moved to `std/start.zig`, and there is no special logic
in the compiler. Instead, the standard library unconditionally imports
the `start.zig` file, which then has a `comptime` block that does the
logic of determining what, if any, start symbols to export. Instead of
`start.zig` being in its own special package, it is just another normal
file that is part of the standard library.

`std.builtin.TestFn` is now part of the standard library rather than
specially generated by the compiler.

src/dump_analysis.cpp

@@ -1216,7 +1216,7 @@ void zig_print_analysis_dump(CodeGen *g, FILE *f, const char *one_indent, const
     jw_end_object(jw);
 
     jw_object_field(jw, "rootPkg");
-    anal_dump_pkg_ref(&ctx, g->root_package);
+    anal_dump_pkg_ref(&ctx, g->main_pkg);
 
     // Poke the functions
     for (size_t i = 0; i < g->fn_defs.length; i += 1) {