clean-ups

[x] use Bos, Fpath, and other libraries for file operations
- Expand tilde: https://github.com/dbuenzli/bos/pull/96
- [x] alternative implementation
[x] use json-data-encoding to handle JSON
[x] don’t resolve symbolic links unless necessary. Exceptions:
- [x] the current working directory

features

[x] Access to units within the same library
[x] Access to built-in libraries that are installed along with OPAM
[x] Locally redirection or vendoring of arbitrary dependencies
[x] Allow different branches of the same remote git repository (though not at the same time)
[x] Support of a public index from the web
[x] Support of a public index from a file
[x] Support of a public index from files installed along with OPAM
[x] Local building environments by default, and co-development of two or more local libraries as an option (e.g., changing both Yuujinchou and algaett at the same time)
[x] Options to work offline (which would turn off git auto-fetch)
[x] Thread/domain-safety