build: Move VERSION into version/version.go

[tsandall]

This way the semantic version is always set. Tested by generating a fake release:

Cutting the release:

(dev)torin:~/src/opa$ make release-patch VERSION=0.24.0 > ~/test.patch
(dev)torin:~/src/opa$ patch -p1 < ~/test.patch
(Stripping trailing CRs from patch; use --binary to disable.)
patching file CHANGELOG.md
(Stripping trailing CRs from patch; use --binary to disable.)
patching file capabilities/v0.24.0.json
(Stripping trailing CRs from patch; use --binary to disable.)
patching file version/version.go
(dev)[*]torin:~/src/opa$ git add .
(dev)[*]torin:~/src/opa$ git commit -a -m 'wip: prepare release'
[dev 077c73b0] wip: prepare release
 3 files changed, 3236 insertions(+), 2 deletions(-)
 create mode 100644 capabilities/v0.24.0.json
(dev)torin:~/src/opa$ git tag v0.24.0

Checking the patches:

(dev)torin:~/src/opa$ head -n 10 CHANGELOG.md
# Change Log

All notable changes to this project will be documented in this file. This
project adheres to [Semantic Versioning](http://semver.org/).

## 0.24.0

### Fixes

- ast: Fix compiler to expand exprs in rule args ([#2649](https://github.com/open-policy-agent/opa/issues/2649))
----------------8<------------------

(dev)torin:~/src/opa$ head -n 10 version/version.go
// Copyright 2016 The OPA Authors.  All rights reserved.
// Use of this source code is governed by an Apache2
// license that can be found in the LICENSE file.

// Package version contains version information that is set at build time.
package version

// Version is the canonical version of OPA.
var Version = "0.24.0"

Building and checking the version:

(dev)torin:~/src/opa$ make build
make[1]: Entering directory '/home/torin/src/opa/wasm'
make: '_obj/opa.wasm' is up to date.
make[1]: Leaving directory '/home/torin/src/opa/wasm'
cp wasm/_obj/opa.wasm internal/compiler/wasm/opa/opa.wasm
CGO_ENABLED=0 GO111MODULE=on GOFLAGS=-mod=vendor go generate
CGO_ENABLED=0 GO111MODULE=on GOFLAGS=-mod=vendor go build -o opa_linux_amd64 -ldflags " -X github.com/open-policy-agent/opa/version.Version=0.24.0 -X github.com/open-policy-agent/opa/version.Vcs=077c73b0 -X github.com/open-policy-agent/opa/version.Timestamp=2020-09-11T15:33:03Z -X github.com/open-policy-agent/opa/version.Hostname=bigbox.localdomain"
(dev)torin:~/src/opa$ ./opa_linux_amd64 version
Version: 0.24.0
Build Commit: 077c73b0
Build Timestamp: 2020-09-11T15:33:03Z
Build Hostname: bigbox.localdomain

Preparing for development of next release:

(dev)torin:~/src/opa$ make dev-patch VERSION=0.25.0 >~/test.patch
(dev)torin:~/src/opa$ patch -p1 < ~/test.patchpatch
(Stripping trailing CRs from patch; use --binary to disable.)
patching file CHANGELOG.md
(Stripping trailing CRs from patch; use --binary to disable.)
patching file version/version.go
(dev)[*]torin:~/src/opa$ git commit -a -m 'wip: prepare dev'
[dev 8f61e3e7] wip: prepare dev
 2 files changed, 3 insertions(+), 1 deletion(-)

Building development version and verifying the version number:

(dev)torin:~/src/opa$ make build
make[1]: Entering directory '/home/torin/src/opa/wasm'
make: '_obj/opa.wasm' is up to date.
make[1]: Leaving directory '/home/torin/src/opa/wasm'
cp wasm/_obj/opa.wasm internal/compiler/wasm/opa/opa.wasm
CGO_ENABLED=0 GO111MODULE=on GOFLAGS=-mod=vendor go generate
CGO_ENABLED=0 GO111MODULE=on GOFLAGS=-mod=vendor go build -o opa_linux_amd64 -ldflags " -X github.com/open-policy-agent/opa/version.Version=0.25.0-dev -X github.com/open-policy-agent/opa/version.Vcs=8f61e3e7 -X github.com/open-policy-agent/opa/version.Timestamp=2020-09-11T15:33:37Z -X github.com/open-policy-agent/opa/version.Hostname=bigbox.localdomain"
(dev)torin:~/src/opa$ ./opa_linux_amd64 version
Version: 0.25.0-dev
Build Commit: 8f61e3e7
Build Timestamp: 2020-09-11T15:33:37Z
Build Hostname: bigbox.localdomain

Verifying the CHANGELOG.md file is up-to-date for development:

(dev)torin:~/src/opa$ head -n 10 CHANGELOG.md
# Change Log

All notable changes to this project will be documented in this file. This
project adheres to [Semantic Versioning](http://semver.org/).

## Unreleased

## 0.24.0

### Fixes