Ngcc perf improvements - #30525

When profiling ngcc it is notable that a large amount of time
is spent dealing with an exception that is thrown (and handled
internally by fs) when checking the existence of a file.

We check file existence a lot in both finding entry-points
and when TS is compiling code. This commit adds a simple
cached `FileSystem`, which wraps a real `FileSystem` delegate.
This will reduce the number of calls through to `fs.exists()` and
`fs.readFile()` on the delegate.

Initial benchmarks indicate that the cache is miss to hit ratio
for `exists()` is about 2:1, which means that we save about 1/3
of the calls to `fs.existsSync()`.

Note that this implements a "non-expiring" cache, so it is not suitable
for a long lived `FileSystem`, where files may be modified externally.
The cache will be updated if a file is changed or moved via
calls to `FileSystem` methods but it will not be aware of changes
to the files system from outside the `FileSystem` service.

For ngcc we must create a new `FileSystem` service
for each run of `mainNgcc` and ensure that all file operations
(including TS compilation) use the `FileSystem` service.
This ensures that it is very unlikely that a file will change
externally during `mainNgcc` processing.

PR Close #30525

…y-point (#30525)

Previously, ngcc had to walk the entire `node_modules` tree looking for
entry-points, even if it only needed to process a single target entry-point
and its dependencies.

This added up to a few seconds to each execution of ngcc, which is noticeable
when being run via the CLI integration.

Now, if an entry-point target is provided, only that target and its entry-points
are considered rather than the whole folder tree.

PR Close #30525

…#30525)

Paths can be mapped directly to files, which was not being taken
into account when computing `basePaths` for the `EntryPointFinder`s.

Now if a `pathMapping` pattern does not exist or is a file, then we try
the containing folder instead.

Fixes #31424

PR Close #30525