Support Go module fallback for nested package paths

[mxr]

I'm trying to set up a mirror for mvdan.cc/sh/v3/cmd/shfmt/ but the actual module is mvdan.cc/sh/v3 (there are a couple tools in the module). go install itself ultimately resolves this by trying to take the longest path and truncating until the module is found:

For example, suppose the go command is looking for a module that provides the package golang.org/x/net/html, and GOPROXY is set to https://corp.example.com,https://proxy.golang.org. The go command may make the following requests:

To https://corp.example.com/ (in parallel):
Request for latest version of golang.org/x/net/html
Request for latest version of golang.org/x/net
Request for latest version of golang.org/x
Request for latest version of golang.org

https://go.dev/ref/mod#resolve-pkg-mod

(I am not implementing the entirety of Go module resolution here, just taking the longest non-404 path, since I imagine this approach works for most cases.)

Tested the change locally by creating a mirror with pre-commit-mirror . --language=golang --package-name=mvdan.cc/sh/v3/cmd/shfmt --types=shell --entry=shfmt --args='-w', then I ran it on a shell file and confirmed the file was formatted.