Add more specific coverage configuration

pyproject.toml:
Set specific source for `toolscoverage.paths` (as we only provide one
module).
Set `tools.coverage.run.relative_files` to true (although it is still
bug riddled and does not seem to work as advertized in regards to xml
output: nedbat/coveragepy#963,
nedbat/coveragepy#1147).
Set `tool.coverage.xml.output`, so we don't have to provide it on the
commandline.

Makefile:
Do not provide an output file to the coverage xml call, as we do that in
configuration now.

Makefile

@@ -17,7 +17,7 @@ fmt:
 test:
 	coverage run
 	coverage report
-	coverage xml -o build/coverage.xml
+	coverage xml
 
 build:
 	./keyringctl -v build

pyproject.toml

@@ -17,8 +17,8 @@ exclude = '''
 )/
 '''
 
-[tool.coverage.path]
-source = "."
+[tool.coverage.paths]
+source = ["libkeyringctl"]
 
 [tool.coverage.report]
 omit = ["tests/*", ".tox/*"]
@@ -29,6 +29,10 @@ show_missing = true
 branch = true
 command_line = "-m pytest --junit-xml=build/junit-report.xml -vv tests/"
 omit = ["tests/*", ".tox/*"]
+relative_files = true
+
+[tool.coverage.xml]
+output = "build/coverage.xml"
 
 [tool.isort]
 profile = "black"