refactor: collapse duplicate XPCommonAPIs constants and simplify crd_types template alias scheme

[jschoombee]

Background

PR #664 migrated the common API imports to github.com/crossplane/crossplane/apis/v2/core/v2. As part of that migration, two constants in pkg/types/reference.go were intentionally kept separate for source-level backward compatibility — but they now hold identical values:

```go
// Both resolve to the same package path since crossplane-runtime v2.3.0
PackagePathXPCommonAPIs = "github.com/crossplane/crossplane/apis/v2/core/v2"
PackagePathXPV2CommonAPIs = "github.com/crossplane/crossplane/apis/v2/core/v2"
```

crd_types.go.tmpl also uses two separate alias template variables (XPCommonAPIsPackageAlias / XPV2CommonAPIsPackageAlias) that today resolve to the same import, because typewriter's UsePackage deduplicates by path.

Problem

The dual-constant, dual-alias scheme creates an invisible invariant: both constants must remain identical or one CRD scope's generated imports will silently diverge without a compile error (until downstream providers regenerate). The naming also implies a distinction between "v1 common APIs" and "v2 common APIs" that no longer exists.

Proposed fix

1. Introduce a single replacement constant (e.g. PackagePathXPCoreAPIs) pointing to github.com/crossplane/crossplane/apis/v2/core/v2.
2. Deprecate PackagePathXPCommonAPIs and PackagePathXPV2CommonAPIs with a // Deprecated: comment redirecting to the new name.
3. Update pkg/pipeline/crd.go to pass the single constant for both template keys.
4. Simplify crd_types.go.tmpl to use one alias variable throughout.

Severity

Not urgent — no correctness issue today. Generated code is correct because UsePackage deduplicates by path. The risk is a future maintainer who diverges only one constant and silently breaks one CRD scope's codegen.

Deferred from #664 to keep that PR focused in size.