Specify configuration modifiers from the CLI

Modifiers can be specified on the command line to override the values set in PACKAGE files or on targets directly. To do so, pass constraint targets via the -m/--modifiers flag:

# Assuming that `//constraints:BUCK` contains the appropriate constraint
# definitions.
buck build :my_target -m //constraints:debug
buck build :my_target -m //constraints:release

Aliases can be used as shorthands (see the setup how-to for more information):

buck build :my_target -m debug

The following shorthand syntax is planned, but is currently unimplemented:

buck2 build :my_target?debug