Proposal: Add modify_classpath to JavaInfo

## 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!

Start by reviewing the module-file patch that exposes modify_classpath and compare it with the current JavaInfo and java_common.merge behavior. Define how excluding jars and adding compile-time or runtime jars should affect the returned JavaInfo, then validate the design before productionizing it and creating a pull request.

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