lang stanza of dune-package should be the same as in dune-project

[Khady]

The dune-package file generated during the installation of a package contains a lang stanza. The version in this stanza is the version of the dune used to compile the package. But the version of dune might be different from the version specified in dune-project.

This is a problem when one wants to downgrade dune in an opam switch. For example if one:

• create a switch with dune 1.9.3 and install devkit
• compile a project
• downgrade dune to 1.8.2 (opam install dune.1.8.2)
• try to compile again the same project

Then one will get an error that looks like:

File "/home/louis/.opam/a/lib/devkit/dune-package", line 1, characters 11-14:
1 | (lang dune 1.9)
              ^^^
Error: Version 1.9 of dune is not supported.
Supported versions:
- 0.0
- 1.0 to 1.8

At this point there is no good solution but to run opam switch reinstall.

The dune-project file of devkit looks like this:

(lang dune 1.5)
(name devkit)

I took devkit as an example, but it is exactly the same thing with any other package using dune as a build system.

[rgrinberg]

I think this is a duplicate of #2147