Reopen #1053

[taigrr]

Vanity import sites don't work.
modernc.org/libc and gopkg.in vanity URLs which redirect downloads don't seem to work the same way with athens that they do with go get -u

I had similar issues downloading golang /x packages, i.e. text, but only if it was not the latest text package. I'm not sure if that's the same or a different issue.

Error Message
If applicable add error message to help explain your problem or console output enclose in triple back-ticks eg,
I didn't want to necrobump, but this is the same issue as #1053

```console

athens  | INFO[8:54PM]: exit status 1: go: module modernc.org/libc/unistd: no matching versions for query "latest" 
athens  |       http-method=GET http-path=/modernc.org/libc/unistd/@v/list kind=Not Found module= operation=download.ListHandler ops=[download.ListHandler pool.List protocol.List vcsLister.List] request-id=d3139f34-a5ef-4043-9809-5e59142cd16d version=

```

To Reproduce
Steps to reproduce the behavior:

1. Self host athens
2. Try to go get modernc.org/libc/unistd

Expected behavior
Athens' download behavior should be the same as the go toolchain command

Environment (please complete the following information):

• OS: Linux 64bit
• Go version : go version go1.23.5 linux/amd64
• Proxy version : gomods/athens:v0.15.4
• Storage (fs/mongodb/s3 etc.) : minio/s3

I am willing to help dig into this a bit as well and attempt to solve it, but only if any changes or fixes have a chance at making it upstream