Complete semantic layer and lexical scopes #5

Build the semantic analyses from PyHIR and stop resolving names inside PyIR lowering.

• semantic/ package with scopes, imports and symbols; move frontend/imports.py there
• ScopeAnalysis: locals, globals, nonlocal, parameters, closures, function/class/comprehension scopes
• Python scoping rules: an assignment anywhere in a function makes the name local for the whole function (a read before a later assignment used to resolve to the module import)
• ImportAnalysis: relative imports resolved from the module name, function-level imports, wildcard imports recorded instead of aborting
• SymbolAnalysis resolving any name in any scope to a canonical identifier through imports and closures, with builtins as python.builtins.<name>; SymbolId accepts non-python. namespaces such as flask.request.args and django.http.HttpRequest.GET
• Scope, import and symbol results are immutable values keyed by ScopeId / SymbolId

Reference: docs/architecture.md §4 Semantic Analysis.

Delivery: #4 (layout, scopes), #6 (imports, symbols)