Proposal: Add modify_classpath to JavaInfo

首先审查公开 modify_classpath 的模块文件补丁，并将其与当前 JavaInfo 和 java_common.merge 的行为进行比较。明确排除 jar 以及添加编译时或运行时 jar 应如何影响返回的 JavaInfo，然后在将其投入生产并创建 pull request 之前验证该设计。

Context

We maintain a large Monorepo for multiple JVM languages that allows for multiple versions of external dependencies.

When JavaInfo objects are merged, the version that ends up on the classpath depends on the order that dependencies were declared in the 'deps' field (and other similar fields, such as exports and runtime_deps).

This can lead to situations where a target may depend directly on MyLibrary-1.0, but a different dependency appearing earlier in 'deps' transitively bring in MyLibrary-2.0. The version resolved on the classpath will be MyLibrary-2.0, despite the desired version being MyLibrary-1.0.

Proposal

Our module file adds a patch file that exposes a new method in JavaInfo called 'modify_classpath'. The method accepts a JavaInfo object and constructs a new JavaInfo object with selected jars pruned from the classpath. This is the method header:

def modify_classpath(java_info, exclude_jars = [], add_compile_jars = [], add_runtime_jars = []):
    """Returns a copy of the given JavaInfo with modified compile and runtime classpaths.

    Enables classpath manipulation: excluding specific jars and/or adding new jars to the compile-time 
    and runtime classpaths. 

    Args:
        java_info: (JavaInfo) The JavaInfo to modify.
        exclude_jars: ([File]) Jars to remove from all classpath fields.
        add_compile_jars: ([File]) Jars to prepend to the compile-time classpath.
        add_runtime_jars: ([File]) Jars to prepend to the runtime classpath.

    Returns:
        (JavaInfo) A new JavaInfo with the modified classpaths.

Our call flow is essentially:

dep_java_infos = [dep[JavaInfo] for dep in ctx.attr.deps if JavaInfo in dep]
merged_java_info = java_common.merge(dep_java_infos)
pruned_java_info = java_common.modify_classpath(merged_java_info, exclude_jars = exclude_jars)

This enables selective exclusion of MyLibrary-2.0 from the merged JavaInfo object, allowing MyLibrary-1.0 to be resolved on the classpath

We have a working proof of concept. We would appreciate high-level feedback before productionizing and creating the PR.

Thank you!