Restart a file's imports when a later arrival lowers its node_modules depth

[Christopher Plieger (cplieger)]

When the files parser reaches a file again at a lower depth, it lowers lowestDepth and sets startSubtasks, but the startedSubTasks guard skips the restart because the first arrival already started the file's imports. Those imports keep the first arrival's depth, so a file reached through node_modules first and then from a root file without passing through node_modules leaves its own imports in sourceFilesFoundSearchingNodeModules, and which arrival comes first depends on scheduling.

Impact: in a monorepo where a package is reached both by a relative import and through a node_modules link, which files count as library files changes between identical runs. The same build can then emit different files (#64498), and an analyzer on the API gives a different answer each run: on vitest, 20 opens of one configuration gave five different counts.

The change restarts the subtasks whenever the depth drops, even when they were already started. Each restart strictly lowers a file's lowestDepth, so the extra work is bounded.

The compiler test symlinkedPackageReachedRelativelyIsNotExternalLibrary has a.ts import ./lib/index and b.ts import lib through a node_modules symlink to lib/. Without the change, lib/util.ts counts as from an external library and its util.js is missing from the emit baseline.

I met this while building deadset-ts, the TypeScript analyzer of deadset, on the TypeScript 7 API. That work hit a small set of related defects, which is why a few reports come from me.

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Fixes #64498

[Copilot]

Copilot review overview

🟢 Approval recommended

The bounded restart logic addresses the root cause and is covered by a focused regression test.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes nondeterministic external-library classification by propagating reduced node_modules depth through already-started imports.

Changes:

• Restarts file subtasks whenever their discovered depth decreases.
• Adds regression coverage for both root-file orderings.

File	Description
tsc/​internal/​compiler/​filesparser.go	Propagates lowered depth through imports.
tsc/​internal/​compiler/​filesparser_test.go	Verifies deterministic classification.

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Copilot review overview

Review effort: Lite
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[Copilot]

Copilot review overview

🟢 Approval recommended

The bounded restart logic directly addresses the scheduling-dependent classification and is covered by a focused compiler test.

Review effort: Balanced
Findings: None