Modularize Terratest into 27 independent modules

Implements complete modularization with no root go.mod, eliminating
dependency bloat and fixing ambiguous import errors.

Fixes #1613

## Changes

- Remove root go.mod
- Add go.work workspace for local development
- Create go.mod for each of 27 modules + internal/lib
- Comprehensive test suite in test/modularization/
- Documentation in MODULARIZATION.md

## Impact

**Before:** Importing any module pulls entire ~50MB library with all cloud SDKs

**After:** Import only what you need (~5MB for terraform module)

## Benefits

- **No ambiguous imports** - Each package has exactly one source
- **Reduced dependencies** - Import only needed modules
- **Independent versioning** - Each module versions separately
- **Cleaner dependency graphs** - No hidden transitive deps from root

## Usage

```go
import "github.com/gruntwork-io/terratest/modules/terraform"
```

```go
require github.com/gruntwork-io/terratest/modules/terraform v0.1.0
```

No replace directives needed - Go fetches from GitHub tags automatically.

## Testing

```bash
cd test/modularization
go test -v ./...
```

Tests validate:
- Dependency isolation
- No ambiguous imports
- All modules build independently
- Workspace builds successfully

## Migration

No breaking changes - import paths stay the same. Consumers automatically
pull only needed modules instead of everything.

Author: james00012

go.mod

@@ -0,0 +1,33 @@
+go 1.24.0
+
+use (
+	./internal/lib
+	./modules/aws
+	./modules/azure
+	./modules/collections
+	./modules/database
+	./modules/dns-helper
+	./modules/docker
+	./modules/environment
+	./modules/files
+	./modules/gcp
+	./modules/git
+	./modules/helm
+	./modules/http-helper
+	./modules/k8s
+	./modules/logger
+	./modules/oci
+	./modules/opa
+	./modules/packer
+	./modules/random
+	./modules/retry
+	./modules/shell
+	./modules/slack
+	./modules/ssh
+	./modules/terraform
+	./modules/terragrunt
+	./modules/test-structure
+	./modules/testing
+	./modules/version-checker
+	./test/modularization
+)