Further module system revamp (fix #659)

To import a module now use:

    # Import module.jq file:
    import "relative/path/to/module" as foo;

    # Use the module's defs as foo::<def-name>

To import a JSON file:

    # Read file.json:
    import "relative/path/to/file" as $foo;
    #
    # Use as $foo::foo

Using `-L` now drops the builtin library path and appends the requested
path to the empty array (or the result of an earlier `-L`).

Support for the `$JQ_LIBRARY_PATH` environment variable has been
removed.

builtin.c

@@ -824,6 +824,21 @@ static jv f_env(jq_state *jq, jv input) {
   return env;
 }
 
+static jv f_get_search_list(jq_state *jq, jv input) {
+  jv_free(input);
+  return jq_get_lib_dirs(jq);
+}
+
+static jv f_get_prog_origin(jq_state *jq, jv input) {
+  jv_free(input);
+  return jq_get_prog_origin(jq);
+}
+
+static jv f_get_jq_origin(jq_state *jq, jv input) {
+  jv_free(input);
+  return jq_get_jq_origin(jq);
+}
+
 static jv f_string_split(jq_state *jq, jv a, jv b) {
   if (jv_get_kind(a) != JV_KIND_STRING || jv_get_kind(b) != JV_KIND_STRING) {
     jv_free(a);
@@ -937,6 +952,9 @@ static const struct cfunction function_list[] = {
   {(cfunction_ptr)f_error, "error", 2},
   {(cfunction_ptr)f_format, "format", 2},
   {(cfunction_ptr)f_env, "env", 1},
+  {(cfunction_ptr)f_get_search_list, "get_search_list", 1},
+  {(cfunction_ptr)f_get_prog_origin, "get_prog_origin", 1},
+  {(cfunction_ptr)f_get_jq_origin, "get_jq_origin", 1},
   {(cfunction_ptr)f_match, "_match_impl", 4},
   {(cfunction_ptr)f_modulemeta, "modulemeta", 1},
   {(cfunction_ptr)f_input, "_input", 1},