Allow renaming Java/Kotlin classes/packages during code generation (#499)

Adds a `rename` field to the Java/Kotlin code generators that allows renaming packages and classes during codegen.

* Add `--rename` flag to CLIs
* Add `rename` property to Gradle API

docs/modules/java-binding/pages/codegen.adoc

@@ -164,13 +164,6 @@ This annotation is required to have `java.lang.annotation.ElementType.TYPE_USE`
 or it may generate code that does not compile.
 ====
 
-.--implement-serializable
-[%collapsible]
-====
-Default: (flag not set) +
-Whether to make generated classes implement `java.io.Serializable`.
-====
-
 Common code generator options:
 
 include::{partialsdir}/cli-codegen-options.adoc[]

docs/modules/java-binding/partials/cli-codegen-options.adoc

@@ -21,3 +21,57 @@ Relative paths are resolved against the working directory.
 Default: (not set) +
 Flag that indicates to generate config classes for use with Spring Boot.
 ====
+
+.--implement-serializable
+[%collapsible]
+====
+Default: (not set) +
+Whether to make generated classes implement `java.io.Serializable`.
+====
+
+.--rename
+[%collapsible]
+====
+Default: (none) +
+Example: `foo.=com.example.foo.` +
+Allows to change default class and package names (derived from Pkl module names) in the generated code.
+
+When you need the generated class or package names to be different from the default names derived from the Pkl module names, you can define a rename mapping, where the key is the original Pkl module name prefix, and the value is its replacement.
+When you do, the generated code's `package` declarations, class names, as well as file locations, will be modified according to this mapping.
+
+The prefixes are replaced literally, which means that dots at the end are important.
+If you want to rename packages only, in most cases, you must ensure that you have an ending dot on both sides of a mapping (except for an empty mapping, if you use it), otherwise you may get unexpected results:
+
+----
+// Assuming the following arguments:
+--rename com.foo.=x  // Dot on the left only
+--rename org.bar=y.  // Dot on the right only
+--rename net.baz=z   // No dots
+
+// The following renames will be made:
+"com.foo.bar" -> "xbar"       // Target prefix merged into the suffix
+"org.bar.baz" -> "y..baz"     // Double dot, invalid name
+"net.baz.qux" -> "z.qux"      // Looks okay, but...
+"net.bazqux"  -> "zqux"       // ...may cut the name in the middle.
+----
+
+When computing the appropriate target name, the longest matching prefix is used:
+
+----
+// Assuming the following arguments:
+--rename com.foo.Main=w.Main
+--rename com.foo.=x.
+--rename com.=y.
+--rename =z.
+
+// The following renames will be made:
+com.foo.Main -> w.Main
+com.foo.bar  -> x.bar
+com.baz.qux  -> y.baz.qux
+org.foo.bar  -> z.org.foo.bar
+----
+
+Repeat this option to define multiple mappings.
+Keys can be arbitrary strings, including an empty string.
+Values must be valid dot-separated fully qualified class name prefixes, possibly terminated by a dot.
+====