Let the terminal decide how to display bold. Fixes#9349.
The issue arose when integrating Homebrew's `Tty` class.
Homebrew's `Tty.white` actually emits escapes for bold.
Fixing `Tty.white` to do what the method name says caused
loss of visibility on light backgrounds.
and corresponding testing_env.rb.
* recast HOMEBREW_BREW_FILE as Hbc.homebrew_executable,
defined in Hbc::Locations
* recast HOMEBREW_REPOSITORY as Hbc.homebrew_repository
* recast HOMEBREW_PREFIX as Hbc.homebrew_prefix
* remove HOMEBREW_LIBRARY
* recast existing Hbc.tapspath as Hbc.homebrew_tapspath
to match new methods
fixes#8705
* convert existing Cask:: namespace to Hbc::
* move Homebrew-fork code under Hbc::
* move freestanding classes such as Tty and TopologicalHash under Hbc::
* recast HOMEBREW_CASK_ constants as HBC_
* modify our Homebrew Formula for backward compatibility
* devscripts and dev docs