Affirm `api/` as the home for installed headers.

Prepare to remove src/ from global build tree include path.

• Create api/legacy for the new legacy_api target.
• Move gmxapi and nblib public header dependencies to api/legacy.

This change is orthogonal to the clarification of the public API. The purpose is to normalize the way we manage installed headers. gmxapi and nblib already provide their public interfaces through files and CMake targets in api/. For the remaining installed libgromacs headers, the legacy_api target provides a transitional home as the modules are individually updated in terms of CMake machinery and as the legacy API functionality is reexamined.

The include sorting no longer works as designed. It should be reimplemented per #3659 (closed) and that is efficient to do after the transition to API-suitable file organization is complete.

Refs #3288