chore(ring_theory): split localization.lean and dedekind_domain.lean (#12206)

These files were rather long and had hundreds-deep dependency graphs. I split them into smaller files with less imports, so that they are easier to build and modify.

Proof nothing was lost:
```bash
$ cat src/ring_theory/localization/*.lean | sort | comm -23 <(sort src/ring_theory/localization.lean) - | grep -E 'lemma|theorem|def|instance|class'
$ cat src/ring_theory/dedekind_domain/*.lean | sort | comm -23 <(sort src/ring_theory/dedekind_domain.lean) - | grep -E 'lemma|theorem|def|instance|class'
giving three equivalent definitions (TODO: and shows that they are equivalent).
```

Zulip thread: https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/Splitting.20.60localization.2Elean.60.20and.20.60dedekind_domain.2Elean

Author: Vierkantor

src/algebra/category/CommRing/instances.lean

@@ -4,7 +4,7 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Andrew Yang
 -/
 import algebra.category.CommRing.basic
-import ring_theory.localization
+import ring_theory.localization.away
 
 /-!
 # Ring-theoretic results in terms of categorical languages

src/algebra/char_p/algebra.lean

@@ -4,7 +4,7 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Jon Eugster, Eric Wieser
 -/
 import algebra.char_p.basic
-import ring_theory.localization
+import ring_theory.localization.fraction_ring
 import algebra.free_algebra
 
 

src/algebraic_geometry/Spec.lean

@@ -6,6 +6,7 @@ Authors: Scott Morrison, Justus Springer
 import algebraic_geometry.locally_ringed_space
 import algebraic_geometry.structure_sheaf
 import data.equiv.transfer_instance
+import ring_theory.localization.localization_localization
 import topology.sheaves.sheaf_condition.sites
 import topology.sheaves.functors
 

src/algebraic_geometry/prime_spectrum/basic.lean

@@ -3,12 +3,13 @@ Copyright (c) 2020 Johan Commelin. All rights reserved.
 Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Johan Commelin
 -/
-import topology.opens
-import ring_theory.ideal.prod
-import ring_theory.ideal.over
-import linear_algebra.finsupp
 import algebra.punit_instances
+import linear_algebra.finsupp
 import ring_theory.nilpotent
+import ring_theory.localization.away
+import ring_theory.ideal.prod
+import ring_theory.ideal.over
+import topology.opens
 import topology.sober
 
 /-!

src/algebraic_geometry/structure_sheaf.lean

@@ -7,7 +7,7 @@ import algebraic_geometry.prime_spectrum.basic
 import algebra.category.CommRing.colimits
 import algebra.category.CommRing.limits
 import topology.sheaves.local_predicate
-import ring_theory.localization
+import ring_theory.localization.at_prime
 import ring_theory.subring.basic
 
 /-!

src/field_theory/ratfunc.lean

@@ -6,7 +6,8 @@ Authors: Anne Baanen
 
 import ring_theory.euclidean_domain
 import ring_theory.laurent_series
-import ring_theory.localization
+import ring_theory.localization.fraction_ring
+import ring_theory.polynomial.content
 
 /-!
 # The field of rational functions

src/linear_algebra/matrix/to_linear_equiv.lean

@@ -6,7 +6,8 @@ Authors: Johannes Hölzl, Patrick Massot, Casper Putz, Anne Baanen
 import linear_algebra.matrix.nondegenerate
 import linear_algebra.matrix.nonsingular_inverse
 import linear_algebra.matrix.to_lin
-import ring_theory.localization
+import ring_theory.localization.fraction_ring
+import ring_theory.localization.integer
 
 /-!
 # Matrices and linear equivalences

src/number_theory/class_number/finite.lean

@@ -5,11 +5,12 @@ Authors: Anne Baanen
 -/
 
 import analysis.special_functions.pow
-import ring_theory.class_group
-import ring_theory.norm
 import linear_algebra.free_module.pid
 import linear_algebra.matrix.absolute_value
 import number_theory.class_number.admissible_absolute_value
+import ring_theory.class_group
+import ring_theory.dedekind_domain.integral_closure
+import ring_theory.norm
 
 /-!
 # Class numbers of global fields

src/number_theory/function_field.lean

@@ -5,7 +5,7 @@ Authors: Anne Baanen, Ashvni Narayanan
 -/
 import field_theory.ratfunc
 import ring_theory.algebraic
-import ring_theory.dedekind_domain
+import ring_theory.dedekind_domain.integral_closure
 import ring_theory.integrally_closed
 
 /-!

src/number_theory/number_field.lean

@@ -7,7 +7,7 @@ Authors: Ashvni Narayanan, Anne Baanen
 import algebra.field.basic
 import data.rat.basic
 import ring_theory.algebraic
-import ring_theory.dedekind_domain
+import ring_theory.dedekind_domain.integral_closure
 import ring_theory.integral_closure
 import ring_theory.polynomial.rational_root