More fin grained keep-build-dir

[rjbou]

Opam no more keep build dir for pinned packages (see #6436). User may want to keep build dir but not for all packages. It can be per package type (from repo/pin/dev/version pin), or even per package.
It can be a new option (as --keep-build-dir and its environment variable), or an information to store in global config or switch config.