Revert recent changes for exportability for package declarations

[nkcsgexi]

We figured that there are several enhancements to make to fully reconcile the exportability of package decls and the ideal user experience of other features. Namely:

1. we need to teach availability checker to treat package decls like SPI decls in terms of requiring explicit availability annotations.
2. we need to allow package decls to reference SPI decls without additional SPI annotations on package decls.

[nkcsgexi]

@swift-ci please smoke test

[nkcsgexi]

@swift-ci please smoke test

[nkcsgexi]

It turns out that I need to revert these two PRs:

• [Sema] Handle package exportability. #73161
• Sema: Report references to missing imports from package extensions through typealiases #73498