There is logic to not measure third-party packages inside configured sources. However, when a (i.e. another) configured source was inside a third-party location, this logic was previously disabled completely.

This caused a problem if a virtual env is set up inside a configured source directory and a configured source package gets installed inside the virtual env. Previously in this case, coverage was measured for all files in the virtual env for the reason described in the previous paragraph.

This commit changes the code to collect all configured source directories inside third-party locations and disable coverage for code in third-party locations only if its not in one of these collected source directories.

* fix: generate xml report packages correctly on windows

* test: check duplicate package names in xml report

* fix: shorten long line in test_xml

We were seeing these failures in the nightly builds:

```
FAILED tests/test_arcs.py::LoopArcTest::test_continue - AssertionError:
  Possible arcs differ: minus is expected, plus is actual
    (-1, 1) # .1
    (1, 2) # 12
    (1, 5) # 15
    (2, 3) # 23
    (3, 1) # 31
  - (4, 1) # 41
    (5, -1) # 5.

  Missing arcs differ: minus is expected, plus is actual
  - (4, 1) # 41
  +

assert False
FAILED tests/test_arcs.py::LoopArcTest::test_break - AssertionError:
  Possible arcs differ: minus is expected, plus is actual
    (-1, 1) # .1
    (1, 2) # 12
    (1, 5) # 15
    (2, 3) # 23
    (3, 5) # 35
  - (4, 1) # 41
    (5, -1) # 5.

  Missing arcs differ: minus is expected, plus is actual
    (1, 5) # 15
  - (4, 1) # 41

assert False
FAILED tests/test_arcs.py::ExceptionArcTest::test_raise_followed_by_statement - AssertionError:
  Possible arcs differ: minus is expected, plus is actual
    (-1, 1) # .1
    (1, 2) # 12
    (2, 3) # 23
    (3, 4) # 34
    (4, 6) # 46
  - (5, 8) # 58
    (6, 7) # 67
    (7, 8) # 78
    (8, -1) # 8.

  Missing arcs differ: minus is expected, plus is actual
  - (5, 8) # 58
  +

assert False
FAILED tests/test_coverage.py::SimpleStatementTest::test_raise_followed_by_statement - AssertionError: [1, 2, 4, 5] != [1, 2, 3, 4, 5]
assert [1, 2, 4, 5] == [1, 2, 3, 4, 5]
  At index 2 diff: 4 != 3
  Right contains one more item: 5
  Full diff:
  - [1, 2, 3, 4, 5]
  ?        ---
  + [1, 2, 4, 5]
FAILED tests/test_coverage.py::SimpleStatementTest::test_break - AssertionError: [1, 2, 3, 5] != [1, 2, 3, 4, 5]
assert [1, 2, 3, 5] == [1, 2, 3, 4, 5]
  At index 3 diff: 5 != 4
  Right contains one more item: 5
  Full diff:
  - [1, 2, 3, 4, 5]
  ?           ---
  + [1, 2, 3, 5]
FAILED tests/test_coverage.py::SimpleStatementTest::test_continue - AssertionError: [1, 2, 3, 5] != [1, 2, 3, 4, 5]
assert [1, 2, 3, 5] == [1, 2, 3, 4, 5]
  At index 3 diff: 5 != 4
  Right contains one more item: 5
  Full diff:
  - [1, 2, 3, 4, 5]
  ?           ---
  + [1, 2, 3, 5]
```

Will fix:

https://github.com/nedbat/coveragepy/security/code-scanning/3
https://github.com/nedbat/coveragepy/security/code-scanning/4

(though tbh, not sure how to close those as fixed?)