Allow suppressing a version conflict

I'm trying to solve a thorny dependency conflict, and it seems like I could really use a way to say "I know you think this is a version conflict, but trust me, it's okay". I'm open to other solutions I may be missing.

Here's a stripped-down version of my install:

```python
maven_install(
artifacts = [
"io.grpc:grpc-netty:1.17.1",
"io.netty:netty-codec-http2:4.1.44.Final",
"io.etcd:jetcd-core:0.3.0",
],
repositories = [
"https://jcenter.bintray.com/",
],
)
```

* `io.grpc:grpc-netty:1.17.1` has a dependency on `io.netty:netty-codec-http2:[4.1.30.Final]`. This is an exact version match -- the `.44` version is a conflict.
* In practice, the later version (`.44`) is perfectly fine, gRPC is just very cautious about compatibility.
* Some other dep (not shown here) needs `io.netty:netty-codec-http2:4.1.44.Final`, the older `.30` version is not okay.
* For app-specific reasons not important here, I can't easily change the version of any of these libraries.

Therefore, I would like a way to declare that `io.grpc:grpc-netty` should depend on `4.1.44.Final` of `io.netty:netty-codec-http2`, regardless of what Maven says.

Now, what I can do is use an exclusion and a custom declaration:
```python
maven_install(
artifacts = [
maven.artifact(
group = "io.grpc",
artifact = "grpc-netty",
version = "1.17.1",
exclusions = [
maven.exclusion("io.netty", "netty-codec-http2"),
],
),
"io.netty:netty-codec-http2:4.1.44.Final",
],
repositories = [
"https://jcenter.bintray.com/",
],
)

# some BUILD.bazel
java_library(
name = "io_grpc_grpc_netty",
exports = ["@maven//:io_grpc_grpc_netty"],
runtime_deps = ["@maven//:io_netty_netty_codec_http2"],
)
```

This works... as long as it's only _my_ code that depends on `grpc-netty`. Whenever I want to use another Maven library that has this as a dep (such as `jetcd-core` listed above), I have to apply the same technique (exclusion, custom library rule), and so on transitively, which gets pretty annoying.

I originally thought I could use `override_targets` for this, but there doesn't seem to be a way to reference the _original_ `@maven//:io_grpc_grpc_netty` target if I override it, so I can't construct my custom target.

Any ideas for a clean solution to this kind of problem?

Start by reproducing the dependency conflict with the maven_install example, then inspect how maven_install, exclusions, and override_targets handle transitive dependencies. Done means providing a supported way to select the requested Netty version without repeating custom exclusions and library rules for every dependent Maven artifact.

Written by the indexing model from the issue text.