[maintenance]: enable generate_opam_files in dune-project

[edwintorok]

Use the simplest solution: copy *.opam to *.opam.template so that we get exactly the same opam files as before, and only declare the names in dune-project.

This will allow us to merge subprojects that already use dune-project to generate opam files (such as xapi-stdext), and we can gradually move opam files to be generated by dune. However first we should drop the opam files that we do not need, which will be partially done in #4939

This is best reviewed using git log -1 --stat which shows that all the .opam.template files are copies:

cohttp-posix.opam                                                       |   2 ++
 cohttp-posix.opam => cohttp-posix.opam.template                         |   0
 dune-project                                                            | 254 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 ezxenstore.opam                                                         |   2 ++
 ezxenstore.opam => ezxenstore.opam.template                             |   0
 forkexec.opam                                                           |   2 ++
 forkexec.opam => forkexec.opam.template                                 |   0
 gzip.opam                                                               |   2 ++
 gzip.opam => gzip.opam.template                                         |   0
[....]

(there are 2 files where there is a 16 line diff, it looks like the old opam files lacked a 'build' rule).

[lindig]

With this approach we can gradually move information into the dune files and give up the templates.