Pruner ignore redundant import.

[daniszam]

Hi! I got some unexpected behaviour when trying to optimise collection proto files.
Seems like imports a.proto won't be removed if any of proto file in collection has a dependency to a.proto

File test/file_with_redundant_dependency.proto contains redundant import import "test/dependency.proto";. After using Pruner, I expected to get a file without this import, but that's not really the case.

    @Test
    public void should_remove_redundant_import() {
        // given
        var loader = new SchemaLoader(FileSystems.getDefault());
        var linker = new Linker(loader, new ErrorCollector(), true, true);
        var testFileLocation = Location.get("<TEST_FILE_LOCATION>");
        loader.initRoots(List.of(testFileLocation), List.of(testFileLocation));
        var fileWithDependency = loader.load("test/file_with_dependency.proto");
        var fileWithRedundantDependency = loader.load("test/file_with_redundant_dependency.proto");

        var schema = linker.link(List.of(fileWithDependency, fileWithRedundantDependency));

        // when
        var optimizedSchema = schema.prune(new PruningRules.Builder()
                .addRoot(List.of(
                        "test.dependency.TestMessage",
                        "test.dependency.TestMessageWithDependency"))
                .build());
        var optimizedProtoFile = optimizedSchema.protoFile("test/file_with_redundant_dependency.proto");

        // then
        assertThat(optimizedProtoFile.toSchema())
                .isEqualTo("""
                        // Proto schema formatted by Wire, do not edit.
                        // Source: test/file_with_redundant_dependency.proto
                         
                        syntax = "proto3";
                         
                        package test.dependency;
                         
                        message TestMessage {
                          string field_1 = 1;
                        }
                        """);
    }

test/file_with_dependency.proto

syntax = "proto3";
package test.dependency;

import "test/dependency.proto";

message TestMessageWithDependency {

  string field_1 = 1;
  Dependency field_2 = 2;
}

test/file_with_redundant_dependency.proto

syntax = "proto3";
package test.dependency;

import "test/dependency.proto";

message TestMessage {

  string field_1 = 1;
}

test/dependency.proto

syntax = "proto3";

package test.dependency;

message Dependency {

  string field_1 = 1;
}

[oldergod]

Could you write a failing test in https://github.com/square/wire/blob/master/wire-schema/src/jvmTest/kotlin/com/squareup/wire/schema/PrunerTest.kt ?