Support for Header-only C++ library

Some C++ libraries can be thin wrappers on top of existing libraries. These thin wrappers can consist solely of header files that require no compilation or linking step. These libraries may still require linktime or runtime dependencies when used.

We could model this as a different kind of linkage and binary/variant.

The issue names no files, tests, or entry points. Start by locating the existing C++ linkage and binary/variant modeling, then review how a header-only wrapper with link-time or runtime dependencies could fit those concepts. Done means the supported model and its expected dependency behavior are defined and covered by tests.

Written by the indexing model from the issue text.