cmd/go: fixes

* Install tools into tool dir always
  (Fixes issue 3049.  Fixes issue 2868.  Fixes issue 2925.)
* Make packages depend on compiler, linker (Fixes issue 3036.)
* Do not recompile packages across roots (Fixes issue 3149.)
* Allow use of binary-only packages (Fixes issue 2775.)
* Avoid duplicate cgo dependencies (Fixes issue 3001.)
* Show less in go get -x.  (Fixes issue 2926.)
* Do not force repo root for existing checkout (Fixes issue 2969.)
* Show full syntax error list always (Fixes issue 2811.)
* Clean arguments before processing (Fixes issue 3034.)
* Add flags for compiler, linker arguments (Fixes issue 2996.)
* Pass flags in make.bash (Fixes issue 3091.)
* Unify build flags, defined in one place.
* Clean up error messages (Fixes issue 3075.  Fixes issue 2923.)
* Support local import paths (Fixes issue 3118.)
* Allow top-level package outside $GOPATH (Fixes issue 3009.)

In addition to these fixes, all commands now take a list of
go files as a way to specify a single package, just as go build and
go run always have.  This means you can:

        go list -json x.go
        go fix x.go
        go vet x.go
        go test x_test.go

Preliminary tests in test.bash.
Mainly testing things that the ordinary build does not.
I don't mind if the script doesn't run on Windows.

I expect that gccgo support is now broken, and I hope that
people will feel free to file issues and/or send CLs to fix it.  :-)

R=golang-dev, dsymonds, r, rogpeppe
CC=golang-dev
https://golang.org/cl/5708054

src/cmd/go/clean.go

@@ -13,7 +13,7 @@ import (
 )
 
 var cmdClean = &Command{
-	UsageLine: "clean [-i] [-r] [-n] [-x] [importpath...]",
+	UsageLine: "clean [-i] [-r] [-n] [-x] [packages]",
 	Short:     "remove object files",
 	Long: `
 Clean removes object files from package source directories.
@@ -50,6 +50,8 @@ The -r flag causes clean to be applied recursively to all the
 dependencies of the packages named by the import paths.
 
 The -x flag causes clean to print remove commands as it executes them.
+
+For more about specifying packages, see 'go help packages'.
 	`,
 }