Allow single builder cycles

[natebosch]

Fixes #3531

When a builder outputs a file with the same extension as a
required_input it will be in a cycle with itself. When builders were
ordered with stronglyConnectedComponents cycles were detected by
looking for a component (cycle) with more than one builder. With the
move to topologicalSort the single builder cycle causes an exception
during the sort. Exclude the builder itself from the list of
dependencies for the graph algorithm call.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:build_runner	2.4.5	ready to publish	build_runner-v2.4.5
package:build_web_compilers	4.0.4-wip	version 4.0.4-wip is pre-release; no publish necessary
package:build	2.4.1-wip	version 2.4.1-wip is pre-release; no publish necessary
package:build_config	1.1.2-dev	version 1.1.2-dev is pre-release; no publish necessary
package:build_modules	5.0.3-wip	version 5.0.3-wip is pre-release; no publish necessary
package:build_daemon	4.0.1-dev	version 4.0.1-dev is pre-release; no publish necessary
package:build_runner_core	7.2.10	already published at pub.dev
package:build_resolvers	2.2.1-wip	version 2.2.1-wip is pre-release; no publish necessary
package:build_test	2.2.0-wip	version 2.2.0-wip is pre-release; no publish necessary
package:scratch_space	1.0.2	already published at pub.dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.