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
