Include LICENSE (or similar) files from dependencies

[BetsyMcPhail]

As dependencies are built, their relevant files are gathered in a licenses
directory. The contents of this directory is then copied to
/wheel/pydrake/doc and is included as part of the wheel.

Resolves #15632

[mwoehlke-kitware]

Please fix the whitespace errors; thanks!

[mwoehlke-kitware]

While the general approach looks okay, I think this is repetitive enough that encapsulating it in a function would probably be useful.

Maybe something like:

extract_license(bzip2
  LICENSE
)

(We can take a list of files, the destination can be derived, and the base directory of the source can be derived from the external-project name.)

[mwoehlke-kitware]

Hrm, we're not using any of these bits, are we?

[mwoehlke-kitware]

Hmm, maybe we should update? I think we're on 7.0.1 because I somewhat arbitrarily picked whatever version is current on Fedora 33 (which in this case is 7.0.1).

[mwoehlke-kitware]

Actually, IIUC there is no need to distribute a license or notice for zlib-licensed software.

Specifically, the license is:

TinyXML-2 is released under the zlib license:

This software is provided 'as-is', without any express or implied warranty. In no event will the authors be held liable for any damages arising from the use of this software.

Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions:

1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.
2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
3. This notice may not be removed or altered from any source distribution.

I don't see anything in there requiring us to redistribute the license notice. We're probably fine just adding a note to this effect.

[mwoehlke-kitware]

Also, FYI, we're using four-space indent, not two-space...

[mwoehlke-kitware]

Here's an attempt (untested) at making a helper function for this:

function(extract_license PROJECT)
    set(command "")
    foreach(file IN LISTS ARGN)
        list(APPEND command
            ${CMAKE_COMMAND} -E copy
            ${CMAKE_BINARY_DIR}/src/${PROJECT}/${file}
            ${LICENSE_DIR}/${PROJECT}/${file}
        )
    endforeach()
    ExternalProject_Add_Step(
        ${PROJECT} CopyLicense
        ${command}
        DEPENDEES install
    )
endfunction()

[mwoehlke-kitware]

Given we aren't actually building anything at present, this seems redundant?