cmd/go: suggest running 'go help gopath' with problematic GOPATH

Suggest running 'go help gopath' when the running 'go install .'
and the folder is outside of GOPATH.

Added link to 'https://golang.org/doc/code.html' in gopath help
for more information.

Example output:

% go install .
go install: no install location for directory f:\x\badmessage outside GOPATH
        please run 'go help gopath' for more information

% go help gopath
... SNIP ...
See https://golang.org/doc/code.html for an example.

Fixes #8457

Change-Id: I0ef6ee3c65bb12af2168eafeb757258aa3835664
Reviewed-on: https://go-review.googlesource.com/9258Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/alldocs.go

@@ -868,6 +868,8 @@ Go searches each directory listed in GOPATH to find source code,
 but new packages are always downloaded into the first directory
 in the list.
 
+See https://golang.org/doc/code.html for an example.
+
 
 Import path syntax
 

src/cmd/go/build.go

@@ -505,7 +505,8 @@ func runInstall(cmd *Command, args []string) {
 			} else if p.ConflictDir != "" {
 				errorf("go install: no install location for %s: hidden by %s", p.Dir, p.ConflictDir)
 			} else {
-				errorf("go install: no install location for directory %s outside GOPATH", p.Dir)
+				errorf("go install: no install location for directory %s outside GOPATH\n"+
+					"\tFor more details see: go help gopath", p.Dir)
 			}
 		}
 	}

src/cmd/go/help.go

@@ -321,6 +321,8 @@ Here's an example directory layout:
 Go searches each directory listed in GOPATH to find source code,
 but new packages are always downloaded into the first directory
 in the list.
+
+See https://golang.org/doc/code.html for an example.
 	`,
 }