Auto merge of #148987 - Zalathar:is-default, r=jieyouxu

bootstrap: Replace `Step::DEFAULT` and `default_condition` with `is_default_step`

- Revised and expanded version of rust-lang/rust#148965

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One of the confusing things about bootstrap's `Step::should_run` is that it combines two loosely-related but non-overlapping responsibilities:
- Registering paths/aliases to decide whether a step should be run in response to paths/aliases passed as explicit command-line arguments
  - When the user invokes `./x test compiler`, this allows bootstrap to know what steps “compiler” should translate into
- Deciding whether a step marked `DEFAULT = true` should actually run or not, when no paths/aliases are explicitly specified
  - When the user invokes `./x test`, this allows bootstrap to know which steps to run by default

This PR therefore splits out the latter of those responsibilities into a dedicated `is_default_step` associated function, which also replaces the existing `DEFAULT` associated constant.

A small number of steps were using `ShouldRun::lazy_default_condition` to specify a condition that should not be run repeatedly if possible, e.g. because it queries external tools. Those steps now perform memoization via fields in `Builder` instead.

r? jieyouxu

Author: bors