Issues with getting +incompatible

**Describe the bug**
A project has `github.com/seaweedfs/goexif v2.0.0+incompatible` set in its modules file. Getting this isn't a problem when you use a standard go modules proxy, however with Athens I get an error. Is this kind of notation not supported?

**Error Message**
```console
DEBUG[8:21PM]: saving github.com@v2.0.0+incompatible to storage... http-method=GET http-path=/github.com/@v/v2.0.0+incompatible.info request-id=3561fb9f-d8f1-4126-934b-0d72adc53597
DEBUG[8:21PM]: saving github.com/seaweedfs@v2.0.0+incompatible to storage... http-method=GET http-path=/github.com/seaweedfs/@v/v2.0.0+incompatible.info request-id=1519658a-68a0-4086-ab7f-e2378bcef537
DEBUG[8:21PM]: saving github.com/seaweedfs/goexif@v2.0.0+incompatible to storage... http-method=GET http-path=/github.com/seaweedfs/goexif/@v/v2.0.0+incompatible.info request-id=d6e0f45a-0ec1-4b01-89ba-8e48653018e5
INFO[8:21PM]: github.com/seaweedfs@v2.0.0+incompatible: invalid github.com import path "github.com/seaweedfs" http-method=GET http-path=/github.com/seaweedfs/@v/v2.0.0+incompatible.info kind=Not Found module=github.com/seaweedfs operation=download.InfoHandler ops=[download.InfoHandler pool.Info protocol.Info protocol.processDownload stash.Pool stasher.Stash stasher.fetchModule goGetFetcher.Fetch module.downloadModule] request-id=1519658a-68a0-4086-ab7f-e2378bcef537 version=v2.0.0+incompatible
INFO[8:21PM]: incoming request http-method=GET http-path=/github.com/seaweedfs/@v/v2.0.0+incompatible.info http-status=404 request-id=1519658a-68a0-4086-ab7f-e2378bcef537
INFO[8:21PM]: github.com@v2.0.0+incompatible: invalid github.com import path "github.com" http-method=GET http-path=/github.com/@v/v2.0.0+incompatible.info kind=Not Found module=github.com operation=download.InfoHandler ops=[download.InfoHandler pool.Info protocol.Info protocol.processDownload stash.Pool stasher.Stash stasher.fetchModule goGetFetcher.Fetch module.downloadModule] request-id=3561fb9f-d8f1-4126-934b-0d72adc53597 version=v2.0.0+incompatible
INFO[8:21PM]: incoming request http-method=GET http-path=/github.com/@v/v2.0.0+incompatible.info http-status=404 request-id=3561fb9f-d8f1-4126-934b-0d72adc53597
INFO[8:21PM]: github.com/seaweedfs/goexif@v2.0.0+incompatible: invalid version: unknown revision v2.0.0 http-method=GET http-path=/github.com/seaweedfs/goexif/@v/v2.0.0+incompatible.info kind=Not Found module=github.com/seaweedfs/goexif operation=download.InfoHandler ops=[download.InfoHandler pool.Info protocol.Info protocol.processDownload stash.Pool stasher.Stash stasher.fetchModule goGetFetcher.Fetch module.downloadModule] request-id=d6e0f45a-0ec1-4b01-89ba-8e48653018e5 version=v2.0.0+incompatible
INFO[8:21PM]: incoming request http-method=GET http-path=/github.com/seaweedfs/goexif/@v/v2.0.0+incompatible.info http-status=404 request-id=d6e0f45a-0ec1-4b01-89ba-8e48653018e5
```

**To Reproduce**
Steps to reproduce the behavior:
Execute `go get github.com/seaweedfs/goexif@v2.0.0+incompatible` while you have your `GOPROXY` environment set to your Athens instance.

**Expected behavior**
For the Athens proxy to successfully fetch, store, and deliver the aforementioned module.

**Environment:**
- OS: Linux 64bit
- Go version: 1.26.7
- Proxy version: v0.18.1
- Storage (fs/mongodb/s3 etc.): disk

Start by reproducing the failure with `go get github.com/seaweedfs/goexif@v2.0.0+incompatible` and `GOPROXY` pointed at Athens. Trace how the proxy handles `.info` requests for versions containing `+incompatible`, then verify that the module can be fetched, stored, and delivered successfully.

Written by the indexing model from the issue text.