tools: Shuffle go code into an idiomatic tree

main packages usually go under a `cmd` directory.
Hoist utility packages to the root `src` directroy so they can be shared.

Change-Id: I0c221f6cd39980f5c202c030cd5134d775533efa
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/50901
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>

tools/src/cmd/fix-tests/main.go

@@ -26,7 +26,7 @@ import (
 	"regexp"
 	"strings"
 
-	"dawn.googlesource.com/tint/tools/src/fix-tests/substr"
+	"dawn.googlesource.com/tint/tools/src/substr"
 )
 
 func main() {

tools/src/cmd/trim-includes/main.go

@@ -35,7 +35,7 @@ import (
 	"sync"
 	"time"
 
-	"dawn.googlesource.com/tint/tools/src/trim-includes/glob"
+	"dawn.googlesource.com/tint/tools/src/glob"
 )
 
 var (

tools/src/glob/glob.go

@@ -23,7 +23,7 @@ import (
 	"os"
 	"path/filepath"
 
-	"dawn.googlesource.com/tint/tools/src/trim-includes/match"
+	"dawn.googlesource.com/tint/tools/src/match"
 )
 
 // Scan walks all files and subdirectories from root, returning those

tools/src/trim-includes/build.sh

@@ -1,11 +0,0 @@
-#!/bin/bash
-
-set -e # Fail on any error.
-
-SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 && pwd )"
-ROOT_DIR="$( cd "${SCRIPT_DIR}/../.." >/dev/null 2>&1 && pwd )"
-
-cd $ROOT_DIR
-autoninja -C out/Debug
-cd $ROOT_DIR/build
-ninja