std.Build: add addAnonymousDependency

This is for bypassing the package manager and directly depending on
another package via the build system. For this to work the anonymous
package must be found on the file system relative to the current
package's build.zig.

lib/std/Build.zig

@@ -1457,7 +1457,26 @@ pub fn dependency(b: *Build, name: []const u8, args: anytype) *Dependency {
     process.exit(1);
 }
 
-fn dependencyInner(
+pub fn anonymousDependency(
+    b: *Build,
+    /// The path to the directory containing the dependency's build.zig file,
+    /// relative to the current package's build.zig.
+    relative_build_root: []const u8,
+    /// A direct `@import` of the build.zig of the dependency.
+    comptime build_zig: type,
+    args: anytype,
+) *Dependency {
+    const arena = b.allocator;
+    const build_root = b.build_root.join(arena, &.{relative_build_root}) catch @panic("OOM");
+    const name = arena.dupe(u8, relative_build_root) catch @panic("OOM");
+    for (name) |*byte| switch (byte.*) {
+        '/', '\\' => byte.* = '.',
+        else => continue,
+    };
+    return dependencyInner(b, name, build_root, build_zig, args);
+}
+
+pub fn dependencyInner(
     b: *Build,
     name: []const u8,
     build_root_string: []const u8,