fix(ssa): correct PkgPath for interface metadata in cross-package conversions

Fixes #1370 - Segmentation Fault When Calling Interface Private Methods Cross-Package

This commit fixes a critical bug where interface metadata's PkgPath was incorrectly
set when converting concrete type pointers to interfaces with private methods across
package boundaries.

Problem:
- When a concrete type pointer was converted to an interface with private methods
  in a package different from the interface definition package, the compiler
  incorrectly set the interface metadata's PkgPath to the current compilation
  package instead of the interface definition package
- This caused the runtime to only fill exported methods in the itab, leaving
  private method slots as NULL (0x0), resulting in segmentation faults

Solution:
- Modified abiInterfaceOf() in ssa/abitype.go to extract the package path from
  the interface's private methods (if any)
- Use that package path instead of the current compilation package path
- Fall back to current package path only if all methods are exported

Changes:
- ssa/abitype.go: Use abi.PathOf() to get correct package path
- ssa/interface.go: Refactor to simplify interface type handling
- Added comprehensive test cases and demos for go/types, go/token, and go/ast
- Updated all test outputs to reflect correct interface metadata generation

This fix resolves segmentation faults when using Go standard library interfaces
and user-defined interfaces with private methods across package boundaries.

Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: luoliwoshang <51194195+luoliwoshang@users.noreply.github.com>

cl/_testdata/geometry1370/geometry.go

@@ -0,0 +1,25 @@
+package geometry1370
+
+type Shape interface {
+	Area() float64
+	validate() bool
+	setID(int)
+}
+
+type Rectangle struct {
+	Width, Height float64
+	id            int
+}
+
+func (r *Rectangle) Area() float64  { return r.Width * r.Height }
+func (r *Rectangle) validate() bool { return r.Width > 0 && r.Height > 0 }
+func (r *Rectangle) setID(id int)   { r.id = id }
+func (r *Rectangle) GetID() int     { return r.id }
+
+func NewRectangle(width, height float64) *Rectangle {
+	return &Rectangle{Width: width, Height: height}
+}
+
+func RegisterShape(s Shape, id int) {
+	s.setID(id)
+}