fix: warn that filterInit removes Init.* when replacing findImportsFromSource

[josaum]

Fixes #149.

findImportsFromSource filtered only exactly Init, but its deprecation message recommends parseImports' + ModuleHeader.filterInit, which removes the whole Init namespace. Anyone following the deprecation message on a file with an explicit Init.* import (e.g. a prelude file importing Init.Data.Nat.Basic, like Mathlib's Mathlib/Data/Fin/Init.lean) gets a silent change in import analysis.

Of the two options in the issue, this PR implements the second: the broad behavior of filterInit is intended — its docstring has explicitly documented "removes every import in the Init namespace (Init itself and Init.*)" since #154 — so the remaining gap is that the deprecation path did not warn that the replacement is not behavior-preserving.

Changes

• The @[deprecated] message on findImportsFromSource now warns that the suggested replacement is not behavior-preserving, and shows how to keep Init.* imports: header.imports.filter (·.module != Init)`.
• The docstrings of filterInit and findImportsFromSource now document the difference explicitly.
• New regression tests in ImportGraphTest/FilterInit.lean pin down both behaviors:
  • filterInit drops an explicit Init.* import from a prelude file;
  • filterInit drops Init itself, including the duplicated implicit Init imports that module-system files can produce, and the implicit Init of a plain header;
  • filtering on ·.module != Init(the deprecated function's behavior) keeps explicitInit.*` imports.

Verification

Locally, both against the branch's pinned toolchain (leanprover/lean4:v4.35.0) and current main's (v4.36.0-rc1): lake build passes, lake exe graph --help works, and lake build ImportGraphTest passes for every target except the pre-existing ImportGraphTest.AnotherFileWithTransitiveImports, which exceeds my sandbox's memory limit (OOM-kill, exit 137) identically on unmodified main, so it is unrelated to this change. The deprecation warning renders as expected. No sorry/admit anywhere.