Fixed PHP release script.

Previously it was releasing content from main/master, not the
release branch!

I fixed this by not checking out a separate copy of protobuf.
Also updated the composer.json so we don't need to rewrite it with
sed.

php/composer.json.dist

@@ -16,8 +16,8 @@
   },
   "autoload": {
     "psr-4": {
-      "Google\\Protobuf\\": "php/src/Google/Protobuf",
-      "GPBMetadata\\Google\\Protobuf\\": "php/src/GPBMetadata/Google/Protobuf"
+      "Google\\Protobuf\\": "src/Google/Protobuf",
+      "GPBMetadata\\Google\\Protobuf\\": "src/GPBMetadata/Google/Protobuf"
     }
   }
 }

php/release.sh

@@ -10,32 +10,21 @@ set -ex
 
 VERSION=$1
 
+rm -rf protobuf-php
 git clone https://github.com/protocolbuffers/protobuf-php.git
-git clone https://github.com/protocolbuffers/protobuf.git
 
 # Clean old files
-pushd protobuf-php
-rm -rf src
-popd
-
-# Checkout the target version
-pushd protobuf/php
-git checkout -b $VERSION
-popd
+rm -rf protobuf-php/src
 
 # Copy files
-pushd protobuf-php
-mv ../protobuf/php/src src
-mv ../protobuf/php/composer.json.dist composer.json
-sed -i 's|php/src|src|g' composer.json
+cp -r php/src protobuf-php
+cp php/composer.json.dist protobuf-php/composer.json
+
+cd protobuf-php
 git add .
 git commit -m "$VERSION"
 if [ $(git tag -l "$VERSION") ]; then
   echo "tag $VERSION already exists"
 else
   git tag "$VERSION"
 fi
-popd
-
-# Clean up
-rm -rf protobuf