Support explicit header-only libraries

To date we have been using static libraries with no source files as
header-only libraries.  Switch to using header_libs to make the user's
expectations clear, in case we need to differentiate the semantics of
static libraries and header-only libraries when we enable transitive
static library dependencies.

Test: mma -j external/llvm
Change-Id: I3ce16df11076b637bd192880e86ec9027738b9e7