Upstream configuration does not take effect

**Is your feature request related to a problem? Please describe.**
I want the private package to be pulled directly from the private code repository. Pull from the upstream server when the third-party package does not exist locally. I don’t know if my configuration is wrong, but it doesn’t take effect.

**Additional context**
version: v0.6.1

ENV:
```shell
GOPROXY=https://mirrors.aliyun.com/goproxy/,direct
```

configuration:
```yaml
GoBinary = "go"
GoEnv = "development"
GoGetWorkers = 40
ProtocolWorkers = 30
LogLevel = "debug"
CloudRuntime = "none"
EnablePprof = false
PprofPort = ":3001"
FilterFile = "/config/filter-file.conf"
Timeout = 300
Port = ":3000"
GlobalEndpoint = "https://mirrors.aliyun.com/goproxy/"
ValidatorHook = ""
PathPrefix = ""
NETRCPath = ""
GithubToken = ""
HGRCPath = ""
TraceExporter = ""
TraceExporterURL = "http://localhost:14268"
StatsExporter = "prometheus"
SumDBs = ["https://sum.golang.org"]
NoSumPatterns = ["code.test.com/*","gitlab.pri.test.com/*"]
DownloadMode = "file:/config/download-hcl.conf"
SingleFlightType = "memory"
StorageType = "minio"
[SingleFlight]
[SingleFlight.Etcd]
Endpoints = "localhost:2379,localhost:22379,localhost:32379"
[SingleFlight.Redis]
Endpoint = "127.0.0.1:6379"
[Storage]
[Storage.Disk]
RootPath = "/path/on/disk"
[Storage.Minio]
Endpoint = "oss"
Key = "key"
Secret = "secret"
EnableSSL = true
Bucket = "athens"
Region = ""
```

download-hcl.conf:

```yaml
downloadURL = "https://mirrors.aliyun.com/goproxy/"
mode = "sync"
download "code.test.com/server/*" {
mode = "sync"
downloadURL = "https://code.test.com"
}

download "gitlab.pri.test.com/*" {
mode = "sync"
downloadURL = "https://gitlab.pri.test.com"

}
```

logs:
```shell
time="2020-12-24T02:29:21Z" level=info msg="exit status 1: go list -m: module github.com/afex/hystrix-go/hystrix: Get https://proxy.golang.org/github.com/afex/hystrix-go/hystrix/@v/list: Forwarding failure\n" http-method=GET http-path=/github.com/afex/hystrix-go/hystrix/@v/list http-url=/github.com/afex/hystrix-go/hystrix/@v/list kind="Not Found" module= operation=download.ListHandler ops="[download.ListHandler pool.List protocol.List vcsLister.List]" version=
time="2020-12-24T02:29:21Z" level=info msg="exit status 1: go list -m: module github.com/afex/hystrix-go: Get https://proxy.golang.org/github.com/afex/hystrix-go/@v/list: Forwarding failure\n" http-method=GET http-path=/github.com/afex/hystrix-go/@v/list http-url=/github.com/afex/hystrix-go/@v/list kind="Not Found" module= operation=download.ListHandler ops="[download.ListHandler pool.List protocol.List vcsLister.List]" version=
time="2020-12-24T02:29:21Z" level=info msg="exit status 1: go list -m: module github.com/afex: Get https://proxy.golang.org/github.com/afex/@v/list: Forwarding failure\n" http-method=GET http-path=/github.com/afex/@v/list http-url=/github.com/afex/@v/list kind="Not Found" module= operation=download.ListHandler ops="[download.ListHandler pool.List protocol.List vcsLister.List]" version=
time="2020-12-24T02:29:21Z" level=info msg="exit status 1: go list -m: module github.com: Get https://proxy.golang.org/github.com/@v/list: Forwarding failure\n" http-method=GET http-path=/github.com/@v/list http-url=/github.com/@v/list kind="Not Found" module= operation=download.ListHandler ops="[download.ListHandler pool.List protocol.List vcsLister.List]" version=
```
Why does the log show that the package is pulled from https://proxy.golang.org instead of https://mirrors.aliyun.com/goproxy/ ？I don’t know why it fails to pull this package.

Start by tracing how GlobalEndpoint, GOPROXY, and download-hcl.conf are loaded and used for module lookup, then compare that path with the logged request to proxy.golang.org. Reproduce the configuration from the issue with version v0.6.1 or the current code. Done means the configured mirror is used and the documented upstream fallback behavior is verified or clearly reported.

Written by the indexing model from the issue text.