Guard against re-fetching an ecosystem dep that's already the top-level project

[ryanmrichard]

Summary

• `nwx_ecosystem_dependency()` only skipped re-fetching a dependency when the `nwx::` alias target already existed (the case where the dep was resolved via `find_package()` against an installed wheel).
• It missed the case where `` is already a bare target because this repo is that dependency's top-level project — built directly via `nwx_library()`, which never defines the `nwx::` alias — and is also, transitively, its own ecosystem dependency. E.g. SCF built with `-DINTEGRATION_TESTING=ON` pulls in the `nwchemex` superproject, which itself depends on `scf`. FetchContent would then re-fetch and redefine the same target, and CMake would fail:

  add_library cannot create target "scf" because another target with the
  same name already exists.
  

  This broke `test_cmake_build` on every platform for NWChemEx/SCF#70 — unrelated to that PR's actual (and successful) macOS pybind11 segfault fix.
• Fix: also short-circuit on `TARGET ${ned_name}` (the bare name), same as the existing `nwx::${ned_name}` branch.

This affects every ecosystem repo that routes through this shared macro (`chemcache`, `chemist`, `integrals`, `parallelzone`, `pluginplay`, `nux`, `nwchemex`, `scf`, `simde`, `tensorwrapper`, `utilities`) whenever it's both a dependency of another ecosystem repo and built with `INTEGRATION_TESTING=ON`.

Test plan

• Minimal standalone CMake repro mirroring `get_dependencies.cmake`'s `FetchContent_MakeAvailable` call:
  • With a pre-existing bare target and the old guard: falls through to `FetchContent`, fails trying to clone a bogus URL (reproduces the bug).
  • With the fix: short-circuits before ever reaching `FetchContent` — no network attempt, clean configure.
  • Regression check: with no pre-existing target (normal, first-time ecosystem fetch), the new branch does not fire — falls through to `FetchContent` exactly as before.
• Once merged, SCF PR Fix broken FetchContent: point chemcache at master #70's `test_cmake_build` should pass on all legs (it FetchContents NWXCMake unpinned from `master`, so this picks up automatically).

🤖 Generated with Claude Code

[github-actions]

🚀 [bumpr] Bumped!
New version:v0.0.19
Changes:v0.0.18...v0.0.19