stage2: add the .debug_line header and associated data types

* the .debug_line header is written properly
 * link.File.Elf gains:
   - SrcFn, which is now a field in Module.Fn
   - SrcFile, which is now a field in Module.Scope.File
 * link.File.Elf gets a whole *Package field rather than only
   root_src_dir_path.
 * the fields first_dbg_line_file and last_dbg_line_file tell where the
   Line Number Program begins and ends, which alows moving files when
   the header gets too big, and allows appending files to the end.
 * codegen is passed a buffer for emitting .debug_line
   Line Number Program opcodes for functions.

See #5963

There is some work-in-progress code here, but I need to go make some
experimental changes to changing how to represent source locations and I
want to do that in a separate commit.

src-self-hosted/Module.zig

@@ -279,6 +279,9 @@ pub const Fn = struct {
     },
     owner_decl: *Decl,
 
+    /// Represents the function in the linked output file.
+    link: link.File.Elf.SrcFn = link.File.Elf.SrcFn.empty,
+
     /// This memory is temporary and points to stack memory for the duration
     /// of Fn analysis.
     pub const Analysis = struct {
@@ -503,6 +506,10 @@ pub const Scope = struct {
         /// Direct children of the file.
         decls: ArrayListUnmanaged(*Decl),
 
+        /// Represents the file in the linker code. The linker code
+        /// uses this field to store data relevant to its purposes.
+        link: link.File.Elf.SrcFile = link.File.Elf.SrcFile.empty,
+
         pub fn unload(self: *File, gpa: *Allocator) void {
             switch (self.status) {
                 .never_loaded,
@@ -792,7 +799,7 @@ pub fn init(gpa: *Allocator, options: InitOptions) !Module {
     const bin_file_dir = options.bin_file_dir orelse std.fs.cwd();
     const bin_file = try link.File.openPath(gpa, bin_file_dir, options.bin_file_path, .{
         .root_name = root_name,
-        .root_src_dir_path = options.root_pkg.root_src_dir_path,
+        .root_pkg = options.root_pkg,
         .target = options.target,
         .output_mode = options.output_mode,
         .link_mode = options.link_mode orelse .Static,