bazel: http_archive doesn't work well for Windows

The following scenario is not well supported on Windows:

• declare an http_archive which contains top-level files
• one of the top-level files in the http_archvie is invoked by a bazel command

The bazel command only succeeds if it’s a clean build, otherwise fails with an error similar to:

ERROR: Source forest creation failed: D:/b/sdlshnzi/execroot/io_bazel/_tmp/2713b7a92d8298dcf32c3b44de75c8b1/root/sb26b24g/execroot/main/external/remote_java_tools/JavaBuilder_deploy.jar (Permission denied)

IIUC this is because Bazel tries to remove all non-symlink directories/files from the execroot tree. On Windows there are no symlinks for regular files and bazel uses copies of the original files, so bazel tries to remove all the files downloaded from http_archive. If they are already in use by some process (this happens if they were already invoked/used by previous bazel runs) bazel fails to delete them.

This issue is a generalization of #7440, which was fixed by placing all top-level files under a directory in the http_archive.