refactor(spx): remove ixgo-aware codegen path

[joeykchen]

Summary

This change removes the ixgo-aware codegen path from spx and keeps embedded package handling on the ispx side only.

What Changed

• remove cmd/spx/internal/gengo
• remove the --ixgogen flag and related branching in spx
• switch spx code generation to embedded xgo/tool
• add a fallback path:
  • prefer go list -m github.com/goplus/xgo
  • if that fails, fall back to system xgo env XGOROOT XGOVERSION
• update install script and release verification to stop referencing the removed ixgogen flow
• update --goenv help text to reflect the current behavior

Why

embeddedpkgs and ixgo-specific handling should not leak into spx.
spx should only care about generating Go code through XGo, while ispx remains the only place that is aware of embedded package behavior.

This also removes the extra codegen implementation inside spx and keeps the generation path closer to XGo's public API.

Validation

• go test ./cmd/spx/... ./pkg/ispx/...

[fennoai]

One regression worth fixing before this lands: the new embedded-xgo path no longer works for freshly scaffolded projects unless xgo is separately installed.

[fennoai]

I found two regressions in the updated generation/build flow that look worth fixing before merge.

[fennoai]

One test-coverage regression in the diff.

[gemini-code-assist]

Code Review

This pull request refactors the code generation logic by removing the --ixgogen flag and transitioning from the xgobuild library to the xgo tool package. It simplifies the build process by consolidating Makefile targets, removing the internal/embeddedpkgs package, and moving embedded package registrations directly into pkg/ispx. Additionally, it introduces a more robust mechanism for resolving the xgo module environment. I have no feedback to provide.

[fennoai]

One regression worth fixing before merging: the main generation target no longer regenerates the relocated pkg/ispx exports.

[fennoai]

Found 2 issues worth addressing.