Compiled Go packages are stored in Unix ar archive files, so you can extract the files within them using ar or go tool pack x pkg.a.
Package files contain compiled code (_go_.o), used by the linker to build binaries, and export data (__.PKGDEF), used by the compiler when building dependent packages. If the package contained assembly files or cgo code, there may be additional .o files, too.
You can use golang.org/x/tools/go/gcexportdata to read export data into a format that can be understood by go/types.
It sounds like you've already found go tool objdump for disassembly. You might also find go tool nm useful for listing symbols.