[BUG] Parser/Linter Include statement order resolution can be improved

[davinci26]

In the following scenario Falco fails but Fastly succeeds:

# depenency1.vcl
sub foo {
   set req.backend = httpbin_org;
}

# main.vcl

backend httpbin_org {
  .connect_timeout = 1s;
  .dynamic = true;
  .port = "443";
  .host = "httpbin.org";
  .first_byte_timeout = 20s;
  .max_connections = 500;
  .between_bytes_timeout = 20s;
  .share_key = "xei5lohleex3Joh5ie5uy7du";
  .ssl = true;
  .ssl_sni_hostname = "httpbin.org";
  .ssl_cert_hostname = "httpbin.org";
  .ssl_check_cert = always;
  .min_tls_version = "1.2";
  .max_tls_version = "1.2";
}

sub vcl_recv {
   call foo;
}

This might be a harder change to resolve but I think the correct approach is instead of linting the VCL separately we should be combining them into a single vcl and then parse/lint that as one. I believe this is the behaviour that fastly would do.

[davinci26]

I think we can go in DFS style parsing where when we see an include statement we push it into the AST