Make a distinction between generated headers and "translated" headers.
This is important to support build-time dependencies when headers are
updated.
Generated headers are those which contain build-time feature
specifications, like `git2_features.h` that are internal to the build
and `experimental.h` that contain API information.
Translated headers are the headers that are in `include/git2`, but may
be translated to have a unique prefix like `incklude/git2-experimental`.
This distinction is important so that the CMakeFiles.txt depend on the
in-tree include files (`src/include`) and the generated header files
_but not_ the translated header files. Otherwise there are two `pack.h`
and it's unclear whether the in-tree build is targeting the one in
`src/include` or the one in the build tree.
Without this, updating an in-tree header file like `pack.h` would not
cause a rebuild of its dependencies.
b1b421fe4b