bootstrap: Replace Step::DEFAULT and default_condition with is_default_step

[Zalathar]

• Revised and expanded version of bootstrap: Split out Step::is_really_default from Step::should_run #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

[Zalathar]

Ignoring the new snapshot tests, the actual git diff --shortstat is more like +511, -307.