accept replacements with branches of github forks

[mohammed90]

Closes #50

[francislavoie]

Have you tested that this works?

xcaddy build --with github.com/caddyserver/forwardproxy@caddy2=github.com/klzgrad/forwardproxy@naive

Actual usecase from someone else who ran into this right after I reported it 😅

I'll try it out myself later when I get to my env, but impl looks about right to me.

[mohammed90]

Have you tested that this works?

xcaddy build --with github.com/caddyserver/forwardproxy@caddy2=github.com/klzgrad/forwardproxy@naive

Actual usecase from someone else who ran into this right after I reported it 😅

I'll try it out myself later when I get to my env, but impl looks about right to me.

Works:

~/projects/caddyserver/xcaddy/cmd/xcaddy ./xcaddy build --with github.com/caddyserver/forwardproxy@caddy2=github.com/klzgrad/forwardproxy@naive --
output ./caddy-fwdproxy
2021/02/26 17:18:17 [INFO] Temporary folder: /Users/mohammed/projects/caddyserver/xcaddy/cmd/xcaddy/buildenv_2021-02-26-1718.581327436
2021/02/26 17:18:17 [INFO] Writing main module: /Users/mohammed/projects/caddyserver/xcaddy/cmd/xcaddy/buildenv_2021-02-26-1718.581327436/main.go
2021/02/26 17:18:17 [INFO] Initializing Go module
2021/02/26 17:18:17 [INFO] exec (timeout=10s): /usr/local/bin/go mod init caddy
go: creating new go.mod: module caddy
go: to add module requirements and sums:
	go mod tidy
2021/02/26 17:18:17 [INFO] Replace github.com/caddyserver/forwardproxy => github.com/klzgrad/forwardproxy@naive
2021/02/26 17:18:17 [INFO] exec (timeout=10s): /usr/local/bin/go mod edit -replace github.com/caddyserver/forwardproxy=github.com/klzgrad/forwardproxy@naive
2021/02/26 17:18:17 [INFO] Pinning versions
2021/02/26 17:18:17 [INFO] exec (timeout=0s): /usr/local/bin/go get -d -v github.com/caddyserver/caddy/v2
go get: added github.com/caddyserver/caddy/v2 v2.3.0
2021/02/26 17:18:24 [INFO] Build environment ready
2021/02/26 17:18:24 [INFO] Building Caddy
2021/02/26 17:18:24 [INFO] exec (timeout=0s): /usr/local/bin/go mod tidy
go: downloading github.com/klzgrad/forwardproxy v0.0.0-20210120121422-9b4a5a242bd6
go: found github.com/caddyserver/forwardproxy in github.com/caddyserver/forwardproxy v0.0.0-00010101000000-000000000000
2021/02/26 17:18:30 [INFO] exec (timeout=0s): /usr/local/bin/go build -o /Users/mohammed/projects/caddyserver/xcaddy/cmd/xcaddy/caddy-fwdproxy -ldflags -w -s -trimpath
2021/02/26 17:19:06 [INFO] Build complete: ./caddy-fwdproxy
2021/02/26 17:19:06 [INFO] Cleaning up temporary folder: /Users/mohammed/projects/caddyserver/xcaddy/cmd/xcaddy/buildenv_2021-02-26-1718.581327436

 ~/projects/caddyserver/xcaddy/cmd/xcaddy ./caddy-fwdproxy list-modules                                                                                                               
admin.api.load
...
http.handlers.forward_proxy

[mholt]

Thanks for this patch! Double-check my logic on my one nit; otherwise this LGTM.