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.

OTP-10523

Earlier, if the connection to a remote cover node was lost, all cover
data was lost and the cover_server on the remote node would die. This
would cause problems if there were cover compiled modules that would
still be executed since they would attempt to write to the no longer
existing ets tables belonging to the cover_server.

This commit changes this behavior so that the cover_server on the
remote node will survive connection loss and continue collecting cover
data. If the connection is re-established then the main node will sync
with the remote node again and cover data will not be lost (unless the
node was down).

OTP-10523

* Added cover:flush(Nodes), which will fetch data from remote nodes
  without stopping cover on those nodes.

* Added cover:get_main_node(), which returns the node name of the main
  node. This is used by test_server to avoid {error,not_main_node}
  when a slave starts another slave (e.g. in test_server's own tests).