[dev.link] cmd/internal/goobj2: separate Autolib from Pkglist in new object file
In CL 196030 we decided to combine the imported package list
(Autolib) and referenced package list (PkgIdx, or Pkglist).
However, in some cases the Autolib list may contain file name,
instead of package path, e.g.
https://go.googlesource.com/go/+/refs/heads/dev.link/src/cmd/compile/internal/gc/main.go#1181
And the linker needs that to locate the file. This mostly happens
with direct invocation of the compiler and linker (i.e., not
through "go build").
Instead of letting the linker make guess of the file name based
on the package path, make Autolib a separate list.