Avoid RequirementSet before reaching the resolver

The RequirementSet implementation conflates requirements incorrectly in
a lot of places. This means the (new) resolver would get incomplete
requirements.

The removes all RequirementSet.add_requirement() calls outside of the
legacy resolver, so the new resolver can get the unmodified list of
requirements specified by the user, allowing for more sophisticated
requirement merging.

src/pip/_internal/cli/req_command.py

@@ -25,7 +25,6 @@ from pip._internal.req.constructors import (
     install_req_from_req_string,
 )
 from pip._internal.req.req_file import parse_requirements
-from pip._internal.req.req_set import RequirementSet
 from pip._internal.self_outdated_check import (
     make_link_collector,
     pip_self_version_check,
@@ -296,15 +295,12 @@ class RequirementCommand(IndexGroupCommand):
         options,          # type: Values
         finder,           # type: PackageFinder
         session,          # type: PipSession
-        check_supported_wheels=True,  # type: bool
     ):
         # type: (...) -> List[InstallRequirement]
         """
         Parse command-line arguments into the corresponding requirements.
         """
-        requirement_set = RequirementSet(
-            check_supported_wheels=check_supported_wheels
-        )
+        requirements = []  # type: List[InstallRequirement]
         for filename in options.constraints:
             for parsed_req in parse_requirements(
                     filename,
@@ -315,7 +311,7 @@ class RequirementCommand(IndexGroupCommand):
                     isolated=options.isolated_mode,
                 )
                 req_to_add.is_direct = True
-                requirement_set.add_requirement(req_to_add)
+                requirements.append(req_to_add)
 
         for req in args:
             req_to_add = install_req_from_line(
@@ -323,7 +319,7 @@ class RequirementCommand(IndexGroupCommand):
                 use_pep517=options.use_pep517,
             )
             req_to_add.is_direct = True
-            requirement_set.add_requirement(req_to_add)
+            requirements.append(req_to_add)
 
         for req in options.editables:
             req_to_add = install_req_from_editable(
@@ -332,7 +328,7 @@ class RequirementCommand(IndexGroupCommand):
                 use_pep517=options.use_pep517,
             )
             req_to_add.is_direct = True
-            requirement_set.add_requirement(req_to_add)
+            requirements.append(req_to_add)
 
         # NOTE: options.require_hashes may be set if --require-hashes is True
         for filename in options.requirements:
@@ -345,10 +341,9 @@ class RequirementCommand(IndexGroupCommand):
                     use_pep517=options.use_pep517
                 )
                 req_to_add.is_direct = True
-                requirement_set.add_requirement(req_to_add)
+                requirements.append(req_to_add)
 
         # If any requirement has hash options, enable hash checking.
-        requirements = requirement_set.all_requirements
         if any(req.has_hash_options for req in requirements):
             options.require_hashes = True
 

src/pip/_internal/commands/install.py

@@ -297,10 +297,7 @@ class InstallCommand(RequirementCommand):
         )
 
         try:
-            reqs = self.get_requirements(
-                args, options, finder, session,
-                check_supported_wheels=not options.target_dir,
-            )
+            reqs = self.get_requirements(args, options, finder, session)
 
             warn_deprecated_install_options(
                 reqs, options.install_options

src/pip/_internal/resolution/legacy/resolver.py

@@ -174,7 +174,7 @@ class Resolver(BaseResolver):
         # based on link type.
         discovered_reqs = []  # type: List[InstallRequirement]
         hash_errors = HashErrors()
-        for req in chain(root_reqs, discovered_reqs):
+        for req in chain(requirement_set.all_requirements, discovered_reqs):
             try:
                 discovered_reqs.extend(self._resolve_one(requirement_set, req))
             except HashError as exc: