Schemas with the same prefix create invalid directory references

[austince]

Given a file tree like:

└── src
    ├── main
    │   └── resources
    │       ├── avro
    │       │   ├── Record1.avsc
    │       │   └── Record2.avsc

And BUILD file like:

filegroup(
    name = "avro-records-srcs",
    srcs = glob([
        "src/main/resources/avro/**/*.avsc",
    ]),
)

avro_java_library(
    name = "schemas",
    srcs = [":avro-records-srcs"],
    strings = True,
)

The avro_gen rule will fail with a FileNotFoundException: src/main/resources/avro/Record

I believe this comes from the _commonprefix function not respecting the directory, but rather just the common string path.