Add package GoDoc for some internal packages (revive)

    internal/maint/maint.go:1:1: package-comments: should have a package comment (revive)
    package maint // import "gotest.tools/v3/internal/maint"
    internal/format/diff.go:1:1: package-comments: should have a package comment (revive)
    package format
    icmd/internal/stub/main.go:1:1: package-comments: should have a package comment (revive)
    package main
    internal/source/defers.go:1:1: package-comments: should have a package comment (revive)
    package source
    internal/assert/assert.go:1:1: package-comments: should have a package comment (revive)
    package assert

Signed-off-by: Sebastiaan van Stijn <[email protected]>

icmd/internal/stub/main.go

@@ -1,3 +1,4 @@
+// Package main produces a test-binary used in tests.
 package main
 
 import (

internal/assert/assert.go

@@ -1,3 +1,4 @@
+// Package assert provides internal utilties for assertions.
 package assert
 
 import (

internal/format/diff.go

@@ -1,3 +1,4 @@
+// Package format provides utilities for formatting diffs and messages.
 package format
 
 import (

internal/maint/maint.go

@@ -1,3 +1,5 @@
+// Package maint implements assert.TestingT for uses outside of test cases,
+// for example, in a TestMain.
 package maint // import "gotest.tools/v3/internal/maint"
 
 import (

internal/source/source.go

@@ -1,3 +1,4 @@
+// Package source provides utilities for handling source-code.
 package source // import "gotest.tools/v3/internal/source"
 
 import (