Restore coverage from the examples

The examples seemed to be running against a different version of
Incremental (possibly from pip's cache?). Ensure they run against
exactly the version of Incremental we're trying to test against by
pre-installing it in an isolated environment.

Since this introduces an isolated environment, also install coverage-p
to enable coverage of the subprocess.

.coveragerc

@@ -5,15 +5,12 @@ source_pkgs = incremental
 source = tests
 branch = True
 parallel = True
-# This must be an absolute path because the example tests
-# run Python processes with alternative working directories.
-data_file = ${TOX_INI_DIR-.}/.coverage
 
 [paths]
 source =
-   src/
-   .tox/*/lib/python*/site-packages/
-   .tox/pypy*/site-packages/
+   src/incremental
+   */src/incremental
+   */site-packages/incremental
 
 [report]
 exclude_lines =

tests/test_examples.py

@@ -8,8 +8,10 @@
 import os
 from importlib import metadata
 from subprocess import run
+from tempfile import TemporaryDirectory
 
 from build import ProjectBuilder
+from build.env import DefaultIsolatedEnv
 from twisted.python.filepath import FilePath
 from twisted.trial.unittest import TestCase
 
@@ -20,11 +22,28 @@
 
 
 def build_and_install(path):  # type: (FilePath) -> None
-    builder = ProjectBuilder(path.path)
-    pkgfile = builder.build("wheel", output_directory=os.environ["PIP_FIND_LINKS"])
-
-    # Force reinstall in case tox reused the venv.
-    run(["pip", "install", "--force-reinstall", pkgfile], check=True)
+    with TemporaryDirectory(prefix="dist") as dist_dir:
+        with DefaultIsolatedEnv(installer="pip") as env:
+            env.install(
+                {
+                    # Install the *exact* version of Incremental under test.
+                    # Otherwise pip might select a different version from
+                    # its cache.
+                    #
+                    # These are formally PEP 508 markers, so we pass a
+                    # file URL.
+                    "incremental @ file://" + os.environ["TOX_PACKAGE"],
+                    # A .pth file so that subprocess generate coverage.
+                    "coverage-p",
+                }
+            )
+            builder = ProjectBuilder.from_isolated_env(env, path.path)
+            env.install(builder.build_system_requires)
+            env.install(builder.get_requires_for_build("wheel", {}))
+            pkgfile = builder.build("wheel", output_directory=dist_dir)
+
+        # Force reinstall in case tox reused the venv.
+        run(["pip", "install", "--force-reinstall", pkgfile], check=True)
 
 
 class ExampleTests(TestCase):

tox.ini

@@ -30,9 +30,10 @@ setenv =
     ; Suppress pointless chatter in the output.
     PIP_DISABLE_PIP_VERSION_CHECK=yes
 
-    tests: TOX_INI_DIR={toxinidir}
     tests: COVERAGE_PROCESS_START={toxinidir}/.coveragerc
-    tests: PIP_FIND_LINKS={env:PIP_FIND_LINKS:{distdir}}
+    ; This must be an absolute path because the example tests
+    ; run Python processes with alternative working directories.
+    tests: COVERAGE_FILE={toxinidir}/.coverage
 
 commands =
     python -V