Support for group of imports without blank lines between groups

[murdos]

To keep compatibility with existing configuration, a special delimiter | is introduced to separate subgroup of imports inside a group. For example "java|javax" will not generate a blank line between the java group and the javax group.

The notion of ImportsGroup has been introduced for clarification, allowing a significant code simplification.

Fixes #1375

Example of usage: defining order as java|javax,org,comwill reorder imports as follow:
Before:

import org.dooda.Didoo;
import java.util.List;
import javax.inject.Inject;
import java.lang.Thread;
import javax.annotation.Nullable;

After:

import java.lang.Thread;
import java.util.List;
import javax.annotation.Nullable;
import javax.inject.Inject;

import org.dooda.Didoo;

[tisonkun]

This line has a bug that only |\\# can be split as { "", "\\#" }, but \\#| will be split as { "\\#" } so configure static subgroup before catch all subgroup is impossible.

[tisonkun]

cc @murdos @nedtwigg

Also, it seems that the doc can be incorrect since the correct syntax is \# instead of \\#.

[tisonkun]

Suggested change

	this.subGroups = Stream.of(importOrder.split("\\" + SUBGROUP_SEPARATOR))
	this.subGroups = Stream.of(importOrder.split("\\" + SUBGROUP_SEPARATOR, -1))

... can help.

[nedtwigg]

Published in plugin-gradle 6.13.0 and plugin-maven 2.30.0.