feat: add --srcdir option

[i386x]

When the repository of Python project on GitHub has the structure

/myproject
    /src
        /myproject
            __init__.py
            somefile.py
    /docs
        ...
    /tests
        ...
    README.md
    tox.ini
    ...

and CI runs tox where tox.ini has set usedevelop = false and it runs tests with pytest --cov=myproject --cov-report=term-missing tests, where myproject refers to myproject package build and installed by tox, the coverage report will look like this:

----------- coverage: platform linux, python 3.6.8-final-0 -----------
Name                                                          Stmts   Miss  Cover   Missing
------------------------------------------------------------------------------------------------
.tox/py36/lib/python3.6/site-packages/myproject/__init__.py       1      0   100%
.tox/py36/lib/python3.6/site-packages/myproject/somefile.py       2      0   100%
------------------------------------------------------------------------------------------------
TOTAL                                                             3      0   100%

The .tox/py36/lib/python3.6/site-packages can be stripped by recently introduced --basedir option. This pull request introduces --srcdir option that specify the package source directory after --basedir option is applied. With this feature, coveralls.io is able to display the content of project files correctly. To demonstrate it:

coveralls

reports the above coverage report to coveralls.io, but since .tox/py36/lib/python3.6/site-packages/myproject/__init__.py is not a valid path on GitHub, coveralls.io displays no coverage details for __init__.py. Running the coveralls as

coveralls --basedir=.tox/py36/lib/python3.6/site-packages --srcdir=src

do the trick and the coverage details for __init__.py are visible in coveralls.io web UI, since src/myproject/__init__.py is an existing path on GitHub repository.

[TheKevJames]

@i386x thanks for the contribution! I appreciate the in-depth description; that use-case makes perfect sense to me.

Your implementation LGTM, I'll get this released as soon as I can!