Rename type xlang -> analyzer (#331)

This is part of renaming the OCaml module `Xlang` to `Analyzer`. This
type name change doesn't change the data format because type names don't
occur in JSON data.

- [x] I ran `make setup && make` to update the generated code after
editing a `.atd` file (TODO: have a CI check)
- [x] I made sure we're still backward compatible with old versions of
the CLI.
For example, the Semgrep backend need to still be able to *consume* data
	  generated by Semgrep 1.50.0.
See
https://atd.readthedocs.io/en/latest/atdgen-tutorial.html#smooth-protocol-upgrades
	  Note that the types related to the semgrep-core JSON output or the
	  semgrep-core RPC do not need to be backward compatible!

semgrep_output_v1.atd

@@ -1867,7 +1867,8 @@ type core_error <python decorator="dataclass(frozen=True)"> = {
    those different files (because ATD does not have a proper module system yet).
 *)
 
-type xlang <ocaml attr="deriving show"> = string wrap <ocaml module="Xlang">
+type analyzer <ocaml attr="deriving show"> =
+  string wrap <ocaml module="Analyzer">
 
 (* A target can either be a traditional code target (now with optional
    associated lockfile) or it can be a lockfile target, which will be used to
@@ -1889,10 +1890,10 @@ type target <ocaml attr="deriving show"> = [
 *)
 type code_target <ocaml attr="deriving show"> = {
   path: fpath (* source file *);
-  (* Must be a valid target analyzer as defined in Xlang.mli.
+  (* Must be a valid target analyzer as defined in Analyzer.mli.
      examples: "ocaml", "python", but also "spacegrep" or "regexp".
   *)
-  analyzer: xlang;
+  analyzer: analyzer;
   products: product list;
   ?lockfile_target: lockfile option;
 }