Deprecate -PspotlessFiles from gradle (still available in maven)

[nedtwigg]

-PspotlessFiles has been deprecated and will be removed. It is slow and error-prone, especially for win/unix cross-platform, and we have better options available now:

• If you are formatting just one file, try the much faster IDE hook
• If you are integrating with git, try the much easier (and faster) ratchetFrom 'origin/master'
• If neither of these work for you, let us know in this PR right here :)

[driv]

SpecificFIlesTest fails in Windows, during gradle check. It uses -DspotlessFiles.

From what I understand it's not worth looking into fixing it?

[nedtwigg]

It is removed from gradle, but still used in maven. Probably

spotless/plugin-maven/src/test/java/com/diffplug/spotless/maven/SpecificFilesTest.java

Lines 24 to 29 in 5d965e7

	String rel = "src/main/java/test" + number + ".java";
	if (absolute) {
	return rootFolder() + "/" + rel;
	} else {
	return rel;
	}

needs to be adjusted by

spotless/lib/src/main/java/com/diffplug/spotless/FileSignature.java

Lines 122 to 125 in 5d965e7

	/** Transforms a unix path to a native one. */
	public static String pathUnixToNative(String pathUnix) {
	return pathUnix.replace('/', File.separatorChar);
	}

We have a pretty big test matrix, but I guess we're not testing the maven plugin on windows...

It's not critical to fix, but if you're already in the weeds it would be nice to fix.

[driv]

There's some issue with patterns from what I can get. Replacing / with \ generates an invalid regex.