coveragerc: support for relative paths to config (not cwd)

[blueyed]

It would be helpful for when coverage gets run in a changed cwd with subprocess handling:

983004.64c4: cwd is now '…/Vcs/pytest-django'
983004.64c4: Tracing '…/Vcs/pytest-django/pytest_django/lazy_django.py'
983023.734b: cwd is now '/tmp/pytest-of-user/pytest-370/test_django_assert_num_queries_output_info0'
983023.734b: Not tracing '…/Vcs/pytest-django/pytest_django/lazy_django.py': falls outside the --include trees

Currently you have to set an environment variable, e.g.

tox.ini:

setenv =
    coverage: COVERAGE_HOME={toxinidir}

coverage.rc:

[run]
source =
  $COVERAGE_HOME/src
  $COVERAGE_HOME/tests

Since this should be working in a backward compatible way, I suggest either
supporting a ./ prefix to enable it, or set some environment variable like
$COVERAGE_CONFIG_DIR before reading the config file.

With the first approach source = . would use the current working directory, but source = ./ would use the directory of the config file.

(via #597 (comment))

Link to current docs, which say that it is relative to cwd: https://coverage.readthedocs.io/en/latest/source.html#execution

[omry]

I am also running into issues cases by me needing to run coverage in a different directory.
another more explicit approach to ./ prefix is to have COVERAGE_HOME set automatically by coverage when it loads the config.

This way something like the config below would work regardless of the presence of the environment variable.

[run]
omit = 
    ${COVERAGE_HOME}/.nox/*
    ${COVERAGE_HOME}/*tests*
    ${COVERAGE_HOME}/plugins/*