Nicer format of inspect for multiple targets

[lefou]

We now separate the output for multiple targets with a blank line.

Before:

± mill inspect main.compile main.compileClasspath
Compiling /home/lefou/work/opensource/mill/build.sc
[1/1] inspect 
main.compile(ScalaModule.scala:195)
    Compiles the current module to generate compiled classfiles/bytecode

Inputs:
    main.scalaVersion
    main.upstreamCompileOutput
    main.allSourceFiles
    main.compileClasspath
    main.javacOptions
    main.scalaOrganization
    main.allScalacOptions
    main.scalaCompilerClasspath
    main.scalacPluginClasspath
main.compileClasspath(JavaModule.scala:306)
    All classfiles and resources from upstream modules and dependencies
    necessary to compile this module

Inputs:
    main.transitiveLocalClasspath
    main.unmanagedClasspath
    main.resolvedIvyDeps

After:

± dev-mill -i inspect main.compile main.compileClasspath
Compiling /home/lefou/work/opensource/mill/build.sc
[1/1] inspect 
main.compile(ScalaModule.scala:195)
    Compiles the current module to generate compiled classfiles/bytecode

Inputs:
    main.scalaVersion
    main.upstreamCompileOutput
    main.allSourceFiles
    main.compileClasspath
    main.javacOptions
    main.scalaOrganization
    main.allScalacOptions
    main.scalaCompilerClasspath
    main.scalacPluginClasspath

main.compileClasspath(JavaModule.scala:306)
    All classfiles and resources from upstream modules and dependencies
    necessary to compile this module

Inputs:
    main.transitiveLocalClasspath
    main.unmanagedClasspath
    main.resolvedIvyDeps