Most notes.xml files will be updated in every release and cause
the kind of the problems described in the previous commit.

Commit 29231033 made cover fall back to compile info if source was not
found in pwd or ../src. This isn't sufficient for source that lies in
subdirectories of ../src when beams and source have been moved since
compilation (eg. install of some OTP applications), so first try finding
source relative to the beam directory.

For example, given a beam path

  /installed/path/to/app-1.0/ebin/root.beam

and a source path

  /compiled/path/to/app/src/subdir/root.erl

then look for

  /installed/path/to/app-1.0/ebin/../src/subdir/root.erl

before the source path.

The change was introduced in commit 29231033.