Support classifiers in artifact overrides

We have an existing dependency on [`org.yaml:snakeyaml:1.26`](https://mvnrepository.com/artifact/org.yaml/snakeyaml/1.26). We want to add a new dependency to [`com.github.javafaker:javafaker:1.0.2`](https://mvnrepository.com/artifact/com.github.javafaker/javafaker/1.0.2).

Unfortunately, the latter has a dependency on `org.yaml:snakeyaml:jar:android` instead of the main artifact. When we use the following repository declaration, it defines both `@maven//:org_yaml_snakeyaml` and `@maven//:org_yaml_snakeyaml_android`, which have conflicting class definitions.

```starlark
maven_install(
artifacts = [
"com.github.javafaker:javafaker:1.0.2",
"org.yaml:snakeyaml:1.26",
],
repositories = [
"https://jcenter.bintray.com/",
],
override_targets = {
# (doesn't work)
# "org.yaml:snakeyaml:jar:android": "@maven//:org_yaml_snakeyaml",
},
)
```

We would like a way to force it to only use a single version, the unclassified one.

The same workaround as in #435 works -- add `org.yaml:snakeyaml` to the other artifact's `exclusions`, and define a custom `java_library` that depends on the desired version.

It seems like an obvious solution would be to support classifiers in `override_targets` as shown above, but maybe there are other, better ways to address this.

Start with the maven_install declaration and override_targets example in the issue, then reproduce the dependency graph involving javafaker and both SnakeYAML artifacts. Trace how classified Maven coordinates become generated targets. Done means a classifier-specific override can select the unclassified artifact without generating conflicting duplicate targets, with coverage for this example.

Written by the indexing model from the issue text.