Create Scheme model and wrap distutils_scheme

Now we have a nicely-typed interface to the calculated scheme,
and some more documentation for the same.

src/pip/_internal/locations.py

@@ -16,6 +16,7 @@ from distutils import sysconfig as distutils_sysconfig
 from distutils.command.install import SCHEME_KEYS  # type: ignore
 from distutils.command.install import install as distutils_install_command
 
+from pip._internal.models.scheme import Scheme
 from pip._internal.utils import appdirs
 from pip._internal.utils.compat import WINDOWS
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING, cast
@@ -153,3 +154,41 @@ def distutils_scheme(
             )
 
     return scheme
+
+
+def get_scheme(
+    dist_name,  # type: str
+    user=False,  # type: bool
+    home=None,  # type: Optional[str]
+    root=None,  # type: Optional[str]
+    isolated=False,  # type: bool
+    prefix=None,  # type: Optional[str]
+):
+    # type: (...) -> Scheme
+    """
+    Get the "scheme" corresponding to the input parameters. The distutils
+    documentation provides the context for the available schemes:
+    https://docs.python.org/3/install/index.html#alternate-installation
+
+    :param dist_name: the name of the package to retrieve the scheme for, used
+        in the headers scheme path
+    :param user: indicates to use the "user" scheme
+    :param home: indicates to use the "home" scheme and provides the base
+        directory for the same
+    :param root: root under which other directories are re-based
+    :param isolated: equivalent to --no-user-cfg, i.e. do not consider
+        ~/.pydistutils.cfg (posix) or ~/pydistutils.cfg (non-posix) for
+        scheme paths
+    :param prefix: indicates to use the "prefix" scheme and provides the
+        base directory for the same
+    """
+    scheme = distutils_scheme(
+        dist_name, user, home, root, isolated, prefix
+    )
+    return Scheme(
+        platlib=scheme["platlib"],
+        purelib=scheme["purelib"],
+        headers=scheme["headers"],
+        scripts=scheme["scripts"],
+        data=scheme["data"],
+    )

src/pip/_internal/models/scheme.py

@@ -0,0 +1,25 @@
+"""
+For types associated with installation schemes.
+
+For a general overview of available schemes and their context, see
+https://docs.python.org/3/install/index.html#alternate-installation.
+"""
+
+
+class Scheme(object):
+    """A Scheme holds paths which are used as the base directories for
+    artifacts associated with a Python package.
+    """
+    def __init__(
+        self,
+        platlib,  # type: str
+        purelib,  # type: str
+        headers,  # type: str
+        scripts,  # type: str
+        data,  # type: str
+    ):
+        self.platlib = platlib
+        self.purelib = purelib
+        self.headers = headers
+        self.scripts = scripts
+        self.data = data