How to set specific GOPATH?

I have my go installed with ubuntu package.
Basics library (fmt etc.) are working correctly.

But I have a real project in /var/www/mygoproject with multiple subfolder ex:

• ./subfolder1
• ./lib1
• ./lib2
• ./subfolder2

subfolderX contain different go applications and libX contain shared code.

I would like, in subfolderX use
import "lib1/package-inside"
but I always get the imported and not used error.

What I have to do ?

edit:
code of /var/www/project/subproject/folder/alpha.go

package main

import (
	"subprojectA/folder/apackage" //doesnt work
    "./apackage" //works but not the cleanest
)

func main() {

	var sr interface{}
	sr = "tmp"

	apackage.Run(sr)
}

The go build system, in the first approximation, resolves import path pth by looking for package named $(basename pth) in directory $GOPATH/src/pth.

It seems to me you're missing the /src/ part.

Useful discussion of GOPATH can be found eg. here, another here