Allow SpringHeaderCheck to load custom files

[vpavic]

Currently, SpringHeaderCheck loads header file from the same package:

spring-javaformat/spring-javaformat/spring-javaformat-checkstyle/src/main/java/io/spring/javaformat/checkstyle/check/SpringHeaderCheck.java

Lines 146 to 147 in 1830776

	String name = "header-" + type + ".txt";
	InputStream inputStream = SpringHeaderCheck.class.getResourceAsStream(name);

Unless I'm missing something, this makes it difficult to use a custom header as it requires users to place the custom header in io/spring/javaformat/checkstyle/check package on their Checkstyle classpath. It would be nice if SpringHeaderCheck would load the custom header from the root of Checkstyle classpath.

To clarify, assuming a Gradle build with default Checkstyle config location of config/checkstyle one need to do the following:

checkstyle {
    toolVersion = '8.12'
    // other config...
}

tasks.withType(Checkstyle) {
    checkstyleClasspath += files('config/checkstyle')
}

And then place the custom header file in config/checkstyle/io/spring/javaformat/checkstyle/check while the desirable would be to simply place it in config/checkstyle.

[philwebb]

Good idea!

[philwebb]

Actually, on review the file isn't really designed to be replaced in that way. It's working by accident currently. I think it would be better if you could specify an external file directly and we ignore the type attribute.