build system: add --watch flag and report source file in InstallFile

This direction is not quite right because it mutates shared state in a
threaded context, so the next commit will need to fix this.

lib/std/Build/Step/InstallFile.zig

@@ -40,6 +40,7 @@ fn make(step: *Step, prog_node: std.Progress.Node) !void {
     _ = prog_node;
     const b = step.owner;
     const install_file: *InstallFile = @fieldParentPtr("step", step);
+    step.addWatchInput(install_file.source);
     const full_src_path = install_file.source.getPath2(b, step);
     const full_dest_path = b.getInstallPath(install_file.dir, install_file.dest_rel_path);
     const cwd = std.fs.cwd();