Rename variables, fields, and functions named Option to OptionDefinition as appropriate in cli parser

[tempoz]

To avoid naming collisions when introducing the Option struct as a parsed form of an option from the command line, make all of the naming in parser.go consistent.

[bduffany]

nit: maybe s/Definition/Schema/g - a little more concise & convenient while still clear?

[tempoz]

I talked with Fabian and he recommended Definition instead of Schema for consistency with the verbiage used by bazel.

[bduffany]

@tempoz sgtm 👍