Delegate most checks to the platform Files.copy.
This changes the exception type thrown in one case,
so document when it happens. Also 'copyTo' no longer
creates target parent directory if it doesn't exist.
#KT-19192
This PR adds most extensions on `java.io.File` in `kotlin.io` to `java.nio.Path`. This includes extensions from `FileReadWrite.kt`, `Utils.kt`, and `FileTreeWalk.kt`.
I attempted to keep the implementations, documentation, and tests as similar as possible to the existing implementations.
I am happy to add, remove, or move to separate PRs any of the functions of this PR.
### `File` extensions that were not added to `Path`
##### `createTempDir`, `createTempFile`
These functions have no `File` parameters, so can't be overloaded. Equivalents exist as `Files.createTempFile()` and `Files.createTempDirectory()`.
##### `startsWith`, `endsWith`, `normalize`, `resolve`, `resolveSibling`
These exist as member functions on `Path`
##### `relativeTo`, `relativeToOrNull`, `relativeToOrSelf`, `toRelativeString`, `toRelativeStringOrNull`
This functionality exists as the `Path.relativize` member function, which is equivalent to `relativeTo`, but with the receiver and parameter flipped. `foo.relativeTo(bar)` is equivalent to `bar.relativize(foo)`. We could potentially add a `relativizeOrNull` extension to make that pattern simpler.
##### `isRooted`
`Path` has a `root` method, so `isRooted` is equivalent to `root != null`
### New extensions
All of the simple boolean attribute checks from `java.nio.Files` were added as extensions on `Path`. These extensions are used commonly enough that it seems worth supporting them. This functionality for `File` is implemented as member methods.
The following `Path` extensions were added:
- `exists`
- `isDirectory`
- `isExecutable`
- `isFile`
- `isHidden`
- `isReadable`
- `isSameFile`
- `isSymbolicLink`
- `isWritable`
Some of these extensions take options that are forwarded to their `Files` method, so all of the extensions were implemented as functions rather than properties for consistency.
Additionally, `Path.listFiles` was added to match the `File.listFiles` method. One motivation for its addition was that it's used several times in the implementation of other file extensions. The way to list directory contents with `java.nio` is via `Files.newDirectoryStream()`, which returns an iterable object that must be closed to avoid leaking resources. It's difficult to use correctly with functions like `map` and `filter`, so this extension was added as a simpler, less error-prone alternative.
### Other changes
I added overloads of several of the read-write that take `OpenOptions` to expose the greater control that `java.nio` introduces. For example, you can use `printWriter(APPEND)` to create a `PrintWriter` that doesn't delete the contents of an existing file.
All the new extensions throw exceptions (such as `NoSuchFileException`) from `java.nio` rather than the copies from `kotlin.io`. The `kotlin.io` copies take `File` objects as parameters, and so aren't compatible with `Path`s.
### Address review comments
- Move varargs parameters to the last position
- Remove PathTreeWalk
#KT-19192
Ilya Gorbunov