move types from builtin to std

 * All the data types from `@import("builtin")` are moved to
  `@import("std").builtin`. The target-related types are moved
  to `std.Target`. This allows the data types to have methods, such as
  `std.Target.current.isDarwin()`.
 * `std.os.windows.subsystem` is moved to
   `std.Target.current.subsystem`.
 * Remove the concept of the panic package from the compiler
   implementation. Instead, `std.builtin.panic` is always the panic
   function. It checks for `@hasDecl(@import("root"), "panic")`,
   or else provides a default implementation.

This is an important step for multibuilds (#3028). Without this change,
the types inside the builtin namespace look like different types, when
trying to merge builds with different target settings. With this change,
Zig can figure out that, e.g., `std.builtin.Os` (the enum type) from one
compilation and `std.builtin.Os` from another compilation are the same
type, even if the target OS value differs.

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