Replace `{dir}` in GEMSRC_CLONE_ROOT with repository url.
For instance, when the repository is https://github.com/foo/bar
and GEMSRC_CLONE_ROOT is set to `/path/to/src/{dir}`,
the destination directory will be `/path/to/src/github.com/foo/bar`.
Add description of `{dir}` in gemsrc_clone_root to README.
Rename a constant from USER_OR_ORG_PLACEHOLDER to DIR_PLACEHOLDER.