cmd/compile: cleanup ir.Package

cmd/compile: cleanup ir.Package

Decls used to contain initializer statement for package-level
variables, but now it only contains ir.Funcs. So we might as well
rename it to Funcs and tighten its type to []*ir.Func.

Similarly, Externs always contains *ir.Names, so its type can be
constrained too.

Change-Id: I85b833e2f83d9d3559ab0ef8ab5d8324f4bc37b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/517855
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>