Allow opam loader to actually use multi-instance opam root

It allow the opam loader to reuse existing opam root with multiple instances. It's the
complementary code that goes with the loader adaptation [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.

[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).