perf: run--watch watchman initialization in background

[gregmagolan]

Sometimes the watchman initialization is slow such as when the directory needs to be re-indexed. There is no reason to block the initial bazel run --norun while waiting for watchman to initialize.

Changes are visible to end-users: no

Test plan

• Covered by existing test cases
• Manual testing; run --watch

[aspect-workflows]

Test

11 test targets passed

Targets

//integration_tests/aspect:configure_test [k8-fastbuild] 1m 52s
//integration_tests/aspect:flags_test [k8-fastbuild]     2m 50s
//integration_tests/aspect:help_test [k8-fastbuild]      1m 48s
//integration_tests/aspect:hints_test [k8-fastbuild]     1m 46s
//integration_tests/aspect:info_test [k8-fastbuild]      1m 31s
//integration_tests/aspect:init_test [k8-fastbuild]      40s
//integration_tests/aspect:lint_test [k8-fastbuild]      1m 1s
//integration_tests/aspect:reenter_test [k8-fastbuild]   21s
//integration_tests/aspect:run_test [k8-fastbuild]       1m 17s
//integration_tests/aspect:version_test [k8-fastbuild]   1m 25s
//pkg/aspect/run:run_test [k8-fastbuild]                 117ms

Total test execution time was 14m 34s. 99 tests (90.0%) were fully cached saving 10s.

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