Change the name of a few variables

[ghost]

The names and casing of variables are not very consistent. I propose to use lowercase for all variables such as OCAML and additionally do the following renaming:

• %{path:file} --> %{dep:file}, I think this way it's clearer what it means
• %{path-no-dep:file} --> %{path:file}
• SCOPE_ROOT --> project_root
• remove %{findlib:...} which is redundant with %{lib:...}

#576 adds a dir_kind in the code that tells us whether the directory contains a jbuild or dune file. We can use that to use the new names only in dune files

[emillon]

We can also use the ~syntax argument passed to the expansion functions (added in #887).

[ghost]

Indeed, it's the same kind of change

[rgrinberg]

@emillon which parts of this PR are you tackling?

Btw, the listing of variables to be renamed is incomplete in this PR. Are you planning on tackling all the renames, or just those in this issue?

[emillon]

I have no precise plans yet, but I was thinking about the path vs deps ones. Maybe we can close this and add bugs for individual items? That'll be easier to track & coordinate.

[rgrinberg]

Up to you. I can handle the renames that aren't mentioned in this ticket for now.

[emillon]

OK, I'll take care of the ones mentioned in here then.

[ghost]

This is done