This //tools/workspace/... package tree contains files related to Drake's
Bazel build system, specifically files relating to downloading and/or compiling
third-party software, as cited by Drake's top-level /WORKSPACE file.
Files directly in the //tools/workspace package are generic helpers,
unrelated to any one third-party library. Files in sub-packages such as
//tools/workspace/eigen are specific to their third-party software; the
sub-package is named to match that software's corresponding entry in the
top-level /WORKSPACE file.
Files named BUILD.bazel denote the package structure within our sub-folders;
in the case of the //tools/workspace/... packages, these are largely just
visibility declarations.
Files named *.BUILD.bazel are Drake-specific build rules for external
libraries or tools that do not natively support Bazel:
https://docs.bazel.build/versions/master/external.html#depending-on-non-bazel-projects
Files named *.bzl are Skylark extensions, usually for workspace rules:
https://docs.bazel.build/versions/master/skylark/concepts.html
https://docs.bazel.build/versions/master/be/workspace.html
Files named *.cps are written the in Common Package Specification language,
which provides the necessary information for a project to be consumed by other
projects via cps2cmake-generated *.cmake files.
TODO(jwnimmer-tri) Write me.