plugins exts BUGFIX find parsed ext instance in submodules

[iliasabk]

Closes: #2575

lyplg_ext_parsed_get_storage() searches only ext->module->parsed->exts for the parsed extension instance. However, top-level extension instances of a submodule are compiled into the main module's compiled extension array (mod_c->exts), while their parsed counterparts remain in the submodule's lysp_submodule::exts. For such instances the lookup fails and the unconditional assert(extp) aborts the process.

Reproducer

module tops {
  yang-version 1.1;
  namespace "urn:tops";
  prefix tp;
  include subs;
  import ietf-yang-structure-ext { prefix sx; }
  sx:structure basetop {
    container c { leaf x { type string; } }
  }
}

submodule subs {
  yang-version 1.1;
  belongs-to tops { prefix tp; }
  import ietf-yang-structure-ext { prefix sx; }
  sx:augment-structure "/tops:basetop/tops:c" {
    leaf y { type string; }
  }
}

$ yanglint -f tree -p . tops.yang
Assertion failed: (extp), function lyplg_ext_parsed_get_storage, file plugins_exts.c, line 580.
Abort trap: 6

The same lookup is reached during compilation when children of a submodule extension instance resolve typedefs or groupings (e.g. an sx:structure with an internal typedef placed in a submodule), so the crash is not limited to printing.

Fix

Search the parsed extension arrays of all included submodules as a fallback when the instance is not found in the main module's parsed extension array. The lookup semantics (first match by extension name) are unchanged.

Regression test

test_submod in tests/utests/extensions/test_structure.c parses a module whose submodule contains a top-level sx:augment-structure and prints its tree with LY_CTX_SET_PRIV_PARSED (which is what yanglint -f tree sets). Before the fix this asserts; with the fix the augment-structure section is printed correctly.