proc/*: minor miscellaneous code cleanups (#2790)

* made Pid a method of Target instead of a method of Process
* changed argument of NewTarget to ProcessInternal, since that's the
  interface that backends have to implement
* removed warnings about ProcessInternal since there is no way for
  users of pkg/proc to access those methods anyway
* made RecordingManipulation an optional interface for backends, Target
  supplies its own dummy implementation when the backend doesn't
* inlined small interfaces that only existed to be inlined in
  proc.Process anyway
* removed unused function findExecutable in the Windows and no-native
  darwin backends
* removed (*EvalScope).EvalVariable, an old synonym for EvalExpression

pkg/proc/core/core.go

@@ -164,8 +164,6 @@ type process struct {
 	breakpoints proc.BreakpointMap
 }
 
-var _ proc.ProcessInternal = &process{}
-
 // thread represents a thread in the core file being debugged.
 type thread struct {
 	th     osThread
@@ -222,7 +220,7 @@ func OpenCore(corePath, exePath string, debugInfoDirs []string) (*proc.Target, e
 		return nil, ErrNoThreads
 	}
 
-	return proc.NewTarget(p, currentThread, proc.NewTargetConfig{
+	return proc.NewTarget(p, p.pid, currentThread, proc.NewTargetConfig{
 		Path:                exePath,
 		DebugInfoDirs:       debugInfoDirs,
 		DisableAsyncPreempt: false,
@@ -459,11 +457,6 @@ func (p *process) Valid() (bool, error) {
 	return true, nil
 }
 
-// Pid returns the process ID of this process.
-func (p *process) Pid() int {
-	return p.pid
-}
-
 // ResumeNotify is a no-op on core files as we cannot
 // control execution.
 func (p *process) ResumeNotify(chan<- struct{}) {