Run: add output directory arguments

This allows running commands that take an output directory argument. The
main thing that was needed for this feature was generated file subpaths,
to allow access to the files in a generated directory. Additionally, a
minor change was required to so that the correct directory is created
for output directory args.

lib/std/Build/Step/WriteFile.zig

@@ -31,7 +31,7 @@ pub const File = struct {
     contents: Contents,
 
     pub fn getPath(file: *File) std.Build.LazyPath {
-        return .{ .generated = &file.generated_file };
+        return .{ .generated = .{ .file = &file.generated_file } };
     }
 };
 
@@ -58,7 +58,7 @@ pub const Directory = struct {
     };
 
     pub fn getPath(dir: *Directory) std.Build.LazyPath {
-        return .{ .generated = &dir.generated_dir };
+        return .{ .generated = .{ .file = &dir.generated_dir } };
     }
 };
 
@@ -181,7 +181,7 @@ pub fn addBytesToSource(write_file: *WriteFile, bytes: []const u8, sub_path: []c
 /// Returns a `LazyPath` representing the base directory that contains all the
 /// files from this `WriteFile`.
 pub fn getDirectory(write_file: *WriteFile) std.Build.LazyPath {
-    return .{ .generated = &write_file.generated_directory };
+    return .{ .generated = .{ .file = &write_file.generated_directory } };
 }
 
 fn maybeUpdateName(write_file: *WriteFile) void {