refactor(ivy): implement a virtual file-system layer in ngtsc + ngcc (#30921)

To improve cross platform support, all file access (and path manipulation)
is now done through a well known interface (`FileSystem`).

For testing a number of `MockFileSystem` implementations are provided.
These provide an in-memory file-system which emulates operating systems
like OS/X, Unix and Windows.

The current file system is always available via the static method,
`FileSystem.getFileSystem()`. This is also used by a number of static
methods on `AbsoluteFsPath` and `PathSegment`, to avoid having to pass
`FileSystem` objects around all the time. The result of this is that one
must be careful to ensure that the file-system has been initialized before
using any of these static methods. To prevent this happening accidentally
the current file system always starts out as an instance of `InvalidFileSystem`,
which will throw an error if any of its methods are called.

You can set the current file-system by calling `FileSystem.setFileSystem()`.
During testing you can call the helper function `initMockFileSystem(os)`
which takes a string name of the OS to emulate, and will also monkey-patch
aspects of the TypeScript library to ensure that TS is also using the
current file-system.

Finally there is the `NgtscCompilerHost` to be used for any TypeScript
compilation, which uses a given file-system.

All tests that interact with the file-system should be tested against each
of the mock file-systems. A series of helpers have been provided to support
such tests:

* `runInEachFileSystem()` - wrap your tests in this helper to run all the
wrapped tests in each of the mock file-systems.
* `addTestFilesToFileSystem()` - use this to add files and their contents
to the mock file system for testing.
* `loadTestFilesFromDisk()` - use this to load a mirror image of files on
disk into the in-memory mock file-system.
* `loadFakeCore()` - use this to load a fake version of `@angular/core`
into the mock file-system.

All ngcc and ngtsc source and tests now use this virtual file-system setup.

PR Close #30921

packages/compiler-cli/test/transformers/program_spec.ts

@@ -5,12 +5,11 @@
  * Use of this source code is governed by an MIT-style license that can be
  * found in the LICENSE file at https://angular.io/license
  */
-
+/// <reference types="node" />
 import * as ng from '@angular/compiler-cli';
 import * as fs from 'fs';
 import * as path from 'path';
 import * as ts from 'typescript';
-
 import {formatDiagnostics} from '../../src/perform_compile';
 import {CompilerHost, EmitFlags, LazyRoute} from '../../src/transformers/api';
 import {checkVersion, createSrcToOutPathMapper} from '../../src/transformers/program';