Provide opam during build

[lefessan]

With the latest version of opam, opam-bin fails because an internal call (to opam show to get the opam file corresponding to the built package) fails because of the global lock (it happens only when creating a switch).

This PR makes opam create an opam description file in the build directory called $OPAMROOT/$SWITCH/.opam-switch/build/$PACKAGE.opam that can be used by hooks, and opam-bin in particular, avoiding the recursive call to opam.

[AltGr]

Thanks!

A few notes:

• please avoid hardcoding specific paths at several places, it's not maintainable; we use the OpamPath module to centralise such definitions. Especially here since it's expected as an interface with plugins.
• maybe a cleaner interface would be to provide the opam file location as a variable to the hook.
• with a variable, since we should already have the opam file stored somewhere, maybe we could even provide its location directly and avoid the extra file write?

[lefessan]

I have done the fix using OpamPath.
For the hook variable, it was not clear to me how to find the location of the opam description file from the place where hook variables are defined.
Moreover, using such a variable means creating an incompatibility between the current and new versions of opam, since the plugin would have to put a hook description that would depend on the version of opam. From that point of view, I would prefer the env. variable or the (current with this PR) implicit file location (opam-bin checks for the existence of the file, and calls opam if the file does not exist, which is compatible with new and old opam versions).

[AltGr]

Suggested changes mostly for style, but there is a reason for not computing the path relative to build_dir, for cases where e.g. --inplace-build was used and build_dir can be outside of the opam-controlled dirs.

[rjbou]

Closing as #4402 merged. Thanks for the PR!