feat(plugin): cross alias packages over RPC as proxy stubs

AliasPackage, CompleteAliasPackage and RootAliasPackage now have generated
proxy stubs, so a package handed to a plugin no longer has to be a real
package: it crosses as the stub matching its concrete variant, answers
getAliasOf / setRootPackageAlias / isRootPackageAlias /
hasSelfVersionRequires, and can be constructed from plugin code.

The setters RootPackageInterface declares are routed through that interface
for every root package instead of through the base Package state. Only the
alias variant needs it -- RootAliasPackage overrides all nine to write
through to the package it aliases -- but a real RootPackage delegates to the
same base state either way, so both take one path.

An alias of an alias has no representation here, so narrowing the
constructor argument to a real package is an explicit error rather than a
silent demotion.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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