Reorder ParameterSource

[kdeldycke]

This reorder the ParameterSource enum by precedence, as suggested in #2879.

It brings an interesting property of being able to check if a value was explicitly provided with that kind of code:

        source = ctx.get_parameter_source("port")
        if source < click.ParameterSource.DEFAULT_MAP:
            ...  # value was explicitly set

Additionally, this PR:

• change enum type from generic Enum to IntEnum
• document the precedence in the Sphinx docs
• expose ParameterSource to the package root