fix(compiler-cli): ensure file_system handles mixed Windows drives (#37959)

The `fs.relative()` method assumed that the file-system is a single tree,
which is not the case in Windows, where you can have multiple drives,
e.g. `C:`, `D:` etc.

This commit changes `fs.relative()` so that it no longer forces the result
to be a `PathSegment` and then flows that refactoring through the rest of
the compiler-cli (and ngcc).  The main difference is that now, in some cases,
we needed to check whether the result is "rooted", i.e an `AbsoluteFsPath`,
rather than a `PathSegment`, before using it.

Fixes #36777

PR Close #37959

packages/compiler-cli/src/ngtsc/file_system/index.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 export {NgtscCompilerHost} from './src/compiler_host';
-export {absoluteFrom, absoluteFromSourceFile, basename, dirname, getFileSystem, isRoot, isRooted, join, relative, relativeFrom, resolve, setFileSystem} from './src/helpers';
+export {absoluteFrom, absoluteFromSourceFile, basename, dirname, getFileSystem, isLocalRelativePath, isRoot, isRooted, join, relative, relativeFrom, resolve, setFileSystem, toRelativeImport} from './src/helpers';
 export {LogicalFileSystem, LogicalProjectPath} from './src/logical';
 export {NodeJSFileSystem} from './src/node_js_file_system';
 export {AbsoluteFsPath, FileStats, FileSystem, PathSegment, PathString} from './src/types';