Remove redundant package aliases from import blocks

I noticed that

```go
package main

import (
	foo "gofumpt-ex/foo"
)

func main() {
	foo.Foo("goodbye, world!")
}
```

wasn't being simplified to

```go
package main

import (
	"gofumpt-ex/foo"
)

func main() {
	foo.Foo("goodbye, world!")
}
```

when the package name of "gofumpt-ex/foo" is "foo". This commit adds
handling for this.

README.md

@@ -229,6 +229,24 @@ import (
 
 </details>
 
+**Redundant package aliases are removed**
+
+<details><summary><i>Example</i></summary>
+
+```go
+import (
+	math "math"
+)
+```
+
+```go
+import (
+	"math"
+)
+```
+
+</details>
+
 **Short case clauses should take a single line**
 
 <details><summary><i>Example</i></summary>

format/format.go

@@ -24,6 +24,7 @@ import (
 	"github.com/google/go-cmp/cmp"
 	"golang.org/x/mod/semver"
 	"golang.org/x/tools/go/ast/astutil"
+	"golang.org/x/tools/go/packages"
 
 	"mvdan.cc/gofumpt/internal/govendor/go/format"
 	"mvdan.cc/gofumpt/internal/version"
@@ -479,7 +480,7 @@ func (f *fumpter) applyPre(c *astutil.Cursor) {
 
 	case *ast.GenDecl:
 		if node.Tok == token.IMPORT && node.Lparen.IsValid() {
-			f.joinStdImports(node)
+			f.tidyImports(node)
 		}
 
 		// Single var declarations shouldn't use parentheses, unless
@@ -930,9 +931,10 @@ func isCgoImport(decl *ast.GenDecl) bool {
 	return v == "C"
 }
 
-// joinStdImports ensures that all standard library imports are together and at
-// the top of the imports list.
-func (f *fumpter) joinStdImports(d *ast.GenDecl) {
+// tidyImports ensures that:
+// - all standard library imports are together and at the top of the imports list, and
+// - any redundant package aliases are removed.
+func (f *fumpter) tidyImports(d *ast.GenDecl) {
 	var std, other []ast.Spec
 	firstGroup := true
 	lastEnd := d.Pos()
@@ -956,6 +958,23 @@ func (f *fumpter) joinStdImports(d *ast.GenDecl) {
 
 	for i, spec := range d.Specs {
 		spec := spec.(*ast.ImportSpec)
+		path, err := strconv.Unquote(spec.Path.Value)
+		if err != nil {
+			panic(err) // should never error
+		}
+
+		// If we import a package with an alias (i.e., if spec.Name is non-nil), and
+		// if the alias is the same as the package's name, then we remove the alias.
+		if spec.Name != nil {
+			packages, err := packages.Load(&packages.Config{Mode: packages.NeedName}, path)
+			if err != nil || len(packages) != 1 {
+				panic(fmt.Sprintf("loading the package %q: %v", path, err)) // should never error
+			}
+			if packages[0].Name == spec.Name.Name {
+				spec.Name = nil
+			}
+		}
+
 		if coms := f.commentsBetween(lastEnd, spec.Pos()); len(coms) > 0 {
 			lastEnd = coms[len(coms)-1].End()
 		}
@@ -966,10 +985,6 @@ func (f *fumpter) joinStdImports(d *ast.GenDecl) {
 			lastEnd = spec.End()
 		}
 
-		path, err := strconv.Unquote(spec.Path.Value)
-		if err != nil {
-			panic(err) // should never error
-		}
 		periodIndex := strings.IndexByte(path, '.')
 		slashIndex := strings.IndexByte(path, '/')
 		switch {

testdata/script/std-imports.txtar

@@ -48,7 +48,7 @@ import (
 import (
 	"foo.local"
 	"foo.local/three"
-	math "math"
+	maths "math"
 )
 
 import (
@@ -99,6 +99,12 @@ import (
 	"k8s.io/apimachinery/pkg/types"
 	"sigs.k8s.io/yaml"
 )
+
+// Redundant package aliases are removed.
+import (
+	filepath "path/filepath"
+	"time"
+)
 -- foo.go.golden --
 package p
 
@@ -135,7 +141,7 @@ import (
 
 // We need to split std vs non-std in this case too.
 import (
-	math "math"
+	maths "math"
 
 	"foo.local"
 	"foo.local/three"
@@ -191,3 +197,9 @@ import (
 
 	"sigs.k8s.io/yaml"
 )
+
+// Redundant package aliases are removed.
+import (
+	"path/filepath"
+	"time"
+)