refactor(ivy): extract import rewriting into a separate interface (#27998)

Currently the ImportManager class handles various rewriting actions of
imports when compiling @angular/core. This is required as code compiled
within @angular/core cannot import from '@angular/core'. To work around
this, imports are rewritten to get core symbols from a particular file,
r3_symbols.ts.

In this refactoring, this rewriting logic is moved out of the ImportManager
and put behind an interface, ImportRewriter. There are three implementers
of the interface:

* NoopImportRewriter, used for compiling all non-core packages.
* R3SymbolsImportRewriter, used when ngtsc compiles @angular/core.
* NgccFlatImportRewriter, used when ngcc compiles @angular/core (special
  logic is needed because ngcc has to rewrite imports in flat bundles
  differently than in non-flat bundles).

This is a precursor to using this rewriting logic in other contexts besides
the ImportManager.

PR Close #27998

packages/core/src/r3_symbols.ts

@@ -14,8 +14,9 @@
  * compiler writes imports to this file.
  *
  * Only a subset of such imports are supported - core is not allowed to declare components or pipes.
- * A check in ngtsc's translator.ts validates this condition. The translator is responsible for
- * translating an external name (prefixed with ɵ) to the internal symbol name as exported below.
+ * A check in ngtsc's `R3SymbolsImportRewriter` validates this condition. The rewriter is only used
+ * when compiling @angular/core and is responsible for translating an external name (prefixed with
+ * ɵ) to the internal symbol name as exported below.
  *
  * The below symbols are used for @Injectable and @NgModule compilation.
  */