Cross compile for Scala 3 WIP

[theon]

Currently stuck on shapeless. As it uses macros we can't use CrossVersion.for3Use2_13

milessabin/shapeless#1043

Current output:

[error] Bad symbolic reference. A signature in /Users/ian/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/com/chuusai/shapeless_2.13/2.3.7/shapeless_2.13-2.3.7.jar(shapeless/ScalaVersionSpecifics.class)
[error] refers to Aliases/T in package scala.reflect.macros which is not available.
[error] It may be completely missing from the current classpath, or the version on
[error] the classpath might be incompatible with the version used when compiling /Users/ian/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/com/chuusai/shapeless_2.13/2.3.7/shapeless_2.13-2.3.7.jar(shapeless/ScalaVersionSpecifics.class).
***** missing reference, looking for Type/T in class Aliases
decls = Scope{}JVM / Compile / compileIncremental 15s
[error] Bad symbolic reference. A signature in /Users/ian/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/com/chuusai/shapeless_2.13/2.3.7/shapeless_2.13-2.3.7.jar(shapeless/ScalaVersionSpecifics.class)
[error] refers to Type/T in class scala.reflect.macros.Aliases which is not available.
[error] It may be completely missing from the current classpath, or the version on
[error] the classpath might be incompatible with the version used when compiling /Users/ian/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/com/chuusai/shapeless_2.13/2.3.7/shapeless_2.13-2.3.7.jar(shapeless/ScalaVersionSpecifics.class).
***** missing reference, looking for Context/T in package scala.reflect.macros.whitebox
decls = Scope{}JVM / Compile / compileIncremental 15s
[error] Bad symbolic reference. A signature in /Users/ian/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/com/chuusai/shapeless_2.13/2.3.7/shapeless_2.13-2.3.7.jar(shapeless/ScalaVersionSpecifics.class)
[error] refers to Context/T in package scala.reflect.macros.whitebox which is not available.
[error] It may be completely missing from the current classpath, or the version on
[error] the classpath might be incompatible with the version used when compiling /Users/ian/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/com/chuusai/shapeless_2.13/2.3.7/shapeless_2.13-2.3.7.jar(shapeless/ScalaVersionSpecifics.class).

[theon]

TODO: If shapeless 2 doesn't make it to Scala 3 soon, investigate duplicating PathPart.scala and QueryKeyValue.scala into scala version specific source directories; a src/main/scala-3/ with this implementation and a src/main/scala-2.13/ with the shapeless implementation

[gregor-i]

@theon I will provide a PR for scala version specific source directories.

[theon]

Superseded by #350