Rename BUILD_DIR to "build" (was "built")
Rename BUILD_DIR to "build" (was "built")

Per discussion in PR #4968, the dirname "build" is widely used for
this purpose.

google/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,mark-friedman/blockly,google/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,mark-friedman/blockly,mark-friedman/blockly,mark-friedman/blockly,google/blockly,google/blockly,mark-friedman/blockly,google/blockly,google/blockly,google/blockly,mark-friedman/blockly,rachel-fenichel/blockly