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
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
Pete Bacon Darwin