refactor(ngcc): move PathMappings to separate file to avoid circular dependency (#36626)

Now that `ngcc/src/ngcc_options` imports `FileWriter` type, there is a
circular dependency detected by the `ts-circular-deps:check` lint check:

```
ngcc/src/ngcc_options.ts
  → ngcc/src/writing/file_writer.ts
  → ngcc/src/packages/entry_point_bundle.ts
  → ngcc/src/ngcc_options.ts
```

This commit moves the `PathMappings` type (and related helpers) to a
separate file to avoid the circular dependency.

NOTE:
The circular dependency was only with taking types into account. There
was no circular dependency for the actual (JS) code.

PR Close #36626

packages/compiler-cli/ngcc/index.ts

@@ -12,7 +12,8 @@ import {AsyncNgccOptions, NgccOptions, SyncNgccOptions} from './src/ngcc_options
 
 export {ConsoleLogger} from './src/logging/console_logger';
 export {Logger, LogLevel} from './src/logging/logger';
-export {AsyncNgccOptions, NgccOptions, PathMappings, SyncNgccOptions} from './src/ngcc_options';
+export {AsyncNgccOptions, NgccOptions, SyncNgccOptions} from './src/ngcc_options';
+export {PathMappings} from './src/path_mappings';
 
 export function process(options: AsyncNgccOptions): Promise<void>;
 export function process(options: SyncNgccOptions): void;