Move legacy_resolve to resolution.legacy.resolver

This gives us a concrete place to put the new resolver code and
resolver-specific modules (`resolution.resolver`).

The reason for another level of hierarchy compared to other modules
is to allow us to move other modules here as they
become implementation details of the legacy resolver. Examples I
have in mind are: `req.req_set`, `req.req_install`,
`req.constructors`, and `operations.prepare`.

src/pip/_internal/cli/req_command.py

@@ -14,7 +14,6 @@ from pip._internal.cli.base_command import Command
 from pip._internal.cli.command_context import CommandContextMixIn
 from pip._internal.exceptions import CommandError, PreviousBuildDirError
 from pip._internal.index.package_finder import PackageFinder
-from pip._internal.legacy_resolve import Resolver
 from pip._internal.models.selection_prefs import SelectionPreferences
 from pip._internal.network.download import Downloader
 from pip._internal.network.session import PipSession
@@ -27,6 +26,7 @@ from pip._internal.req.constructors import (
 )
 from pip._internal.req.req_file import parse_requirements
 from pip._internal.req.req_set import RequirementSet
+from pip._internal.resolution.legacy.resolver import Resolver
 from pip._internal.self_outdated_check import (
     make_link_collector,
     pip_self_version_check,

tests/unit/test_legacy_resolve.py

@@ -7,7 +7,9 @@ from pip._internal.exceptions import (
     NoneMetadataError,
     UnsupportedPythonVersion,
 )
-from pip._internal.legacy_resolve import _check_dist_requires_python
+from pip._internal.resolution.legacy.resolver import (
+    _check_dist_requires_python,
+)
 from pip._internal.utils.packaging import get_requires_python
 
 

tests/unit/test_req.py

@@ -18,7 +18,6 @@ from pip._internal.exceptions import (
     InvalidWheelFilename,
     PreviousBuildDirError,
 )
-from pip._internal.legacy_resolve import Resolver
 from pip._internal.network.download import Downloader
 from pip._internal.network.session import PipSession
 from pip._internal.operations.prepare import RequirementPreparer
@@ -38,6 +37,7 @@ from pip._internal.req.req_file import (
     handle_requirement_line,
 )
 from pip._internal.req.req_tracker import get_requirement_tracker
+from pip._internal.resolution.legacy.resolver import Resolver
 from pip._internal.utils.urls import path_to_url
 from tests.lib import assert_raises_regexp, make_test_finder, requirements_file