I'm on Windows, and I got it by giving command go help gopath to cmd, and read the bold text in the instruction, 
that is if code you wnat to install is at ..BaseDir...\SomeProject\src\basic\set, the GOPATH should not be the same location as code, it should be just Base Project DIR: ..BaseDir...\SomeProject.
The GOPATH environment variable lists places to look for Go code. On
  Unix, the value is a colon-separated string. On Windows, the value is
  a semicolon-separated string. On Plan 9, the value is a list.
If the environment variable is unset, GOPATH defaults to a
  subdirectory named "go" in the user's home directory ($HOME/go on
  Unix, %USERPROFILE%\go on Windows), unless that directory holds a Go
  distribution. Run "go env GOPATH" to see the current GOPATH.
See https://golang.org/wiki/SettingGOPATH to set a custom GOPATH.
Each directory listed in GOPATH must have a prescribed structure:
The src directory holds source code. The path below src determines the
  import path or executable name.
The pkg directory holds installed package objects. As in the Go tree,
  each target operating system and architecture pair has its own
  subdirectory of pkg (pkg/GOOS_GOARCH).
If DIR is a directory listed in the GOPATH, a package with source in
  DIR/src/foo/bar can be imported as "foo/bar" and has its compiled form
  installed to "DIR/pkg/GOOS_GOARCH/foo/bar.a".
The bin directory holds compiled commands. Each command is named for
  its source directory, but only the final element, not the entire path.
  That is, the command with source in DIR/src/foo/quux is installed into
  DIR/bin/quux, not DIR/bin/foo/quux. The "foo/" prefix is stripped so
  that you can add DIR/bin to your PATH to get at the installed
  commands. If the GOBIN environment variable is set, commands are
  installed to the directory it names instead of DIR/bin. GOBIN must be
  an absolute path.
Here's an example directory layout:
GOPATH=/home/user/go
/home/user/go/
    src/
        foo/
            bar/               (go code in package bar)
                x.go
            quux/              (go code in package main)
                y.go
    bin/
        quux                   (installed command)
    pkg/
        linux_amd64/
            foo/
                bar.a          (installed package object)
..........
if GOPATH has been set to Base Project DIR and still has this problem, in windows you can try to set GOBIN as  Base Project DIR\bin or %GOPATH%\bin.