This PR aims to track the ongoing development of a new procedure that would help users to upgrade their projects to dune_lang >=2.0. The hope is that a vast majority of projects can be migrated in a mostly automated way. After the upgrade users will be prompted with a list of hard-to-migrate things that they should check manually.

Progression

Doc updates

• self_build_stubs_archive does not exists since 2.0
• c_names / c_flags / etc do not exist since 2.0

Breaking changes that should be automatically handled

• Get rid of ad-hoc rules for guessing the version. Dune now only
  relies on the version written in the dune-project file and no
  longer read VERSION or similar files.
• The action field in the alias stanza is not available starting lang dune 2.0. The alias field in the rule stanza is a replacement.
• Set up formatting rules by default. They can be configured through a new
  (formatting) stanza in dune-project.
  💡 The upgrader adds (formatting disabled) when the project was not formatted. It adds (formatted (enabled_for ...)) when needed.
• *Change the default modes field of executables to (mode exe). If
  one wants to build a bytecode program, it now needs to be explicitly
  requested via (modes byte exe). *
  💡 The upgrader adds (modes byte exe) to executables without explicit mode.
• Error out when a preprocessor_deps field is present but not
  preprocess field is. It is a warning with Dune 1.x projects.
  💡 The upgrader removes preprocessor_deps field when there is no preprocess field or if the chosen preprocess is no_preprocessing
• no_keep_locs is a no-op for projects that use lang dune older than 2.0. In
  projects where the language is at least 2.0, the field is now forbidden.
• c_flags, c_names and cxx_names are now supported in executable and executables stanzas.
  Note: this feature has been subsequently extended into a separate foreign_stubs field.
  💡 In libraries and executables stanzas, the upgrader replaces the fields c(xx)_names and c(xx)_flags by an appropriate foreign_stubs field.

Breaking changes that the user should be aware of

• Drop support for jbuild and jbuild-ignore files.
• *mli only modules must now be explicitly declared. This was previously a
  warning and is now an error. *
• self_build_stubs_archive was deleted in version 2.0 of the dune language
• Stop installing the ocaml-syntax-shims binary. In order to use
  future_syntax, one now need to depend on the ocaml-syntax-shims
  package.
• Actions which introduce targets where new targets are forbidden (e.g.
  preprocessing) are now an error instead of a warning.
• Stop installing the ocaml-syntax-shims binary. In order to use
  future_syntax, one now need to depend on the ocaml-syntax-shims
  package.
• Do not put the <package>.install files in the source tree unless -p or
  --promote-install-files is passed on the command line
• Library names are now validated in a strict fashion. Previously, invalid names
  would be allowed for unwrapped libraries
• Stricter validation of file names in select. The file names of conditional
  sources must match the prefix and the extension of the resultant filename.
• Modules filtered out from the module list via the Ordered Set Language must
  now be actual modules.
• Stub names are no longer allowed relative paths. This was previously a warning
  and is now an error.
• In (diff? x y) action, require x to exist and register a
  dependency on that file.

Non-breaking changes

• The rest of the changelog 2.0.0 section.