Add m2e support

[kwin]

This closes #1413

Please DO NOT FORCE PUSH. Don't worry about messy history, it's easier to do code review if we can tell what happened after the review, and force pushing breaks that.

Please make sure that your PR allows edits from maintainers. Sometimes its faster for us to just fix something than it is to describe how to fix it.

After creating the PR, please add a commit that adds a bullet-point under the [Unreleased] section of CHANGES.md, plugin-gradle/CHANGES.md, and plugin-maven/CHANGES.md which includes:

• a summary of the change
• either
  • a link to the issue you are resolving (for small changes)
  • a link to the PR you just created (for big changes likely to have discussion)

If your change only affects a build plugin, and not the lib, then you only need to update the plugin-foo/CHANGES.md for that plugin.

If your change affects lib in an end-user-visible way (fixing a bug, updating a version) then you need to update CHANGES.md for both the lib and all build plugins. Users of a build plugin shouldn't have to refer to lib to see changes that affect them.

This makes it easier for the maintainers to quickly release your changes :)

[nedtwigg]

Thanks, great PR! I've got one minor nit below, address that and then I'd be happy to merge. @lutovich do you want to take a look? No worries if you're too busy, if I don't hear from you in a week I'll assume you're okay with it.

[nedtwigg]

Should this be && instead of ||?

[lutovich]

LGTM! I added one nit suggestion and have two questions just to better understand the change (I have no experience with m2e):

1. Is my understanding correct that a simple non-Eclipse maven build will inject a DefaultBuildContext that will not affect up-to-date checking?
2. What BuildContext will be injected when a build happens in Eclipse?

[lutovich]

Looks like there's a compilation error:

> Task :plugin-maven:compileJava FAILED
/home/circleci/project/plugin-maven/src/main/java/com/diffplug/spotless/maven/incremental/UpToDateChecker.java:52: error: no suitable method found for hasDelta(Path)
				if (buildContext.hasDelta(file)) {
				                ^
    method BuildContext.hasDelta(String) is not applicable
      (argument mismatch; Path cannot be converted to String)
    method BuildContext.hasDelta(File) is not applicable
      (argument mismatch; Path cannot be converted to File)
    method BuildContext.hasDelta(List) is not applicable
      (argument mismatch; Path cannot be converted to List)
1 error

[kwin]

LGTM! I added one nit suggestion and have two questions just to better understand the change (I have no experience with m2e):

1. Is my understanding correct that a simple non-Eclipse maven build will inject a DefaultBuildContext that will not affect up-to-date checking?

Yes, compare also with https://github.com/codehaus-plexus/plexus-build-api/blob/master/README.md#default-implementation

2. What BuildContext will be injected when a build happens in Eclipse?

The one from https://github.com/eclipse-m2e/m2e-core/blob/d4c976bb9c8d0aabb029030c3c4f3430af8a8c9e/org.eclipse.m2e.core/src/org/eclipse/m2e/core/internal/builder/plexusbuildapi/EclipseBuildContext.java

[nedtwigg]

Released in plugin-maven 2.29.0.