Go 1.21 more fixes

[eh-steve]

Fixes:

• Don't rebuild partial packages purely because of their inittask if the inittask wasn't found in the host binary - it was probably eliminated due to being a no-op.
• Improve firstmodule type registration by including unexported types (reflect.Type.Name() returns an empty string if TFlagNamed is not set, which is now the case for unexported types)
• Only mark firstmodule types as unresolved symbols if their unreachable methods need to be reachable in JIT code (avoids rebuilding of more packages than needed)
• Fix compilation failure on 1.21 go:info.runtime.pinnedTypemaps: relocation target go:info.[]map[github.com/eh-steve/goloader.typeOff]*github.com/eh-steve/goloader._type not defined
  Cherrypicks:
  pkujhd@ac97855
  pkujhd@22941f1
  pkujhd@2fbc174