Signed measures require around 500 more imports than unsigned measures, because they use Bochner integration. As there is no particular reason to use signed measures to prove the Lebesgue decomposition theorem, we use unsigned measures instead.
This PR does not have a big impact on imports, since the file also contains other lemmas about integrability. The effect will be seen in a future PR splitting those out.

* origin/master: (133 commits)
  chore(1000): remove nonexistent fields (#20493)
  chore(CategoryTheory/Adjunction.Basic, CategoryTheory/Equivalence): change definition of `Adjunction.comp` and `Equivalence.trans` (#20490)
  feat(Asymptotics): add `Asymptotics.*.*Prod` lemmas (#20458)
  feat: a conditional kernel is almost everywhere a probability measure (#20430)
  feat: if `f` is a measurable group hom, then every point has a neighborhood `s` such that `f '' s` is bounded (#20304)
  feat: `Ico`, `Ioc`, and `Ioo` are not closed or compact (#20479)
  chore: drop redundant hypothesis in `Module.Dual.eq_of_preReflection_mapsTo` (#20492)
  feat(FaaDiBruno): derive `DecidableEq` (#20482)
  chore(SetTheory/Ordinal/Basic): `{x // x < y}` → `Iio y` (#20413)
  chore: generalize `FormalMultilinearSeries.ofScalars_norm` to `Seminormed` (#20351)
  chore(MvPolynomial/Localization): golf using TensorProduct (#20309)
  chore(Combinatorics/Enumerative/Partition): auto-derive DecidableEq (#20277)
  chore(CategoryTheory): make relevant parameters explicit in `Arrow.homMk`. (#20259)
  feat: add `IsStarNormal (↑x⁻¹ : R)` instance for `x : Rˣ` (#20091)
  fix: Stop cancelling builds of master (#20435)
  chore: update Mathlib dependencies 2025-01-05 (#20485)
  feat(SetTheory/Cardinal/Arithmetic): miscellaneous cardinality lemmas (#18933)
  chore: bump toolchain to v4.16.0-rc1, and merge bump/v4.16.0 (#20464)
  chore: bot validates lean-toolchain on bump/v4.X.0 branches (#20478)
  feat: shorthand lemmas for the L1 norm (#20383)
  chore: remove unnecessary adaptation notes (#20467)
  chore(Algebra/Category/MonCat/Colimits): remove smallness condition (#20473)
  chore(SetTheory/Ordinal/Arithmetic): `IsLeftCancelAdd Ordinal` (#19888)
  feat(Algebra): more on `OrthogonalIdempotents` (#18195)
  feat(Radical): `(radical a).natDegree ≤ a.natDegree` (#20468)
  feat(Polynomial): `(a^n)' = 0` iff `a' = 0` when `n` doesn't divide the characteristic (#20190)
  feat(Algebra/Lie): add Lie's theorem (#13480)
  chore(RingTheory/Generators): make algebra instance a def (#14265)
  feat(Topology/Group): Lemmas about profinite group (#20282)
  feat: the empty set is a topological basis iff the space is empty (#20441)
  chore: make uniqueness of conditionally complete linearly ordered archimedean fields independent of the construction of `Real` (#20242)
  chore: `inherit_doc`s for notations (#20376)
  chore: split AEEqOfIntegral into two files, one for each integral type (#20405)
  chore: split Kernel/MeasurableIntegral (#20427)
  feat(MeasureTheory/Group/Measure): add ContinuousMulEquiv.isHaarMeasure_map (#20469)
  fix(Mathlib.Tactic.CC.Datatypes): `cc` raises panic (#20422)
  feat(Probability/Moments): add lemmas about moment generating functions (#19886)
  feat(Algebra/Order/AddGroupWithTop): lemmas about LinearOrderedAddCommGroupWithTop (#18954)
  chore: bump toolchain to v4.15.0 (#20461)
  chore: update Mathlib dependencies 2025-01-04 (#20463)
  fix: make `Prod` projection delaborators respond to options, add option to disable (#20455)
  chore(Algebra): Improve attribute generation (#20451)
  feat(Polynomial): `(C a * p).degree = p.degree` if `a * p.leadingCoeff ≠ 0` (#20446)
  feat: add `ENNReal.finStronglyMeasurable_of_measurable` (#20404)
  doc(Algebra/Lie/Weights/Basic): fix typo (#20450)
  chore(1000): remove `identifiers` (#20445)
  feat: add sumPiEquivProdPi and piUnique (#20195)
  feat: add fst_compProd_apply (#20429)
  chore: use unsigned measures for Lebesgue decomposition (#20400)
  feat: Two lemmas on divisibility and coprimality of `expand` (#20143)
  ...