It's an attempt to actually allow the opam loader to reuse existing opam root
with multiple instances. It's the complementary code for the lister work [1].
As the `opam show` (cli) [2] version currently packaged does not support the means to
enclose the metadata extraction per opam instance (when sharing the same opam root), we
actually work around this by opening internal details to opam.
It's a prototype that needs completion.
[1] D6316
[2] `opam show` is currently the interface we are using to extract and list information
about a package. It does work on standalone opam root folder but it comes short when
sharing multiple instances within one opam root (for now).