Fixing mocked modules coverage handling

[diogommartins]

Currently, elixir modules mocked using the meck library may leave a trace in the modules list that excoveralls tries to cover which won't have a source key. As mocks, It is expected that those modules wouldn't have a corresponding source file, but excoveralls currently doesn't handle this case. This pull request addresses this issue by improving the bypass logic of modules without a source file.

Examples of the return of module_info/1

Unmocked, compiled and executed module

[
  version: '7.4',
  options: [:no_spawn_compiler_process, :from_core, :no_auto_import],
  source: '/Users/diogo.martins/dev/excoveralls/test_missing.ex'
]

Mocked module

[version: '7.4', options: []]

The mocked module scenario starts breaking when whe assume that every module has a :source at Keyword.get(info, :source). It returns nil and has_compile_info?/1 tries to call File.exists?/1 with nil, resulting in the following stacktrace:

Randomized with seed 319344
** (FunctionClauseError) no function clause matching in IO.chardata_to_string/1    
    
    The following arguments were given to IO.chardata_to_string/1:
    
        # 1
        nil
    
    Attempted function clauses (showing 2 out of 2):
    
        def chardata_to_string(+string+) when -is_binary(string)-
        def chardata_to_string(+list+) when -is_list(list)-
    
    (elixir) lib/io.ex:461: IO.chardata_to_string/1
    (elixir) lib/file.ex:214: File.exists?/2
    lib/excoveralls/cover.ex:34: ExCoveralls.Cover.has_compile_info?/1
    (elixir) lib/enum.ex:2934: Enum.filter_list/2
    (elixir) lib/enum.ex:2935: Enum.filter_list/2
    lib/excoveralls.ex:39: ExCoveralls.execute/2
    (mix) lib/mix/tasks/test.ex:390: Mix.Tasks.Test.run/1
    (mix) lib/mix/task.ex:331: Mix.Task.run_task/3
    (mix) lib/mix/project.ex:348: Mix.Project.in_project/4
    (elixir) lib/file.ex:1506: File.cd!/2
    (mix) lib/mix/task.ex:431: anonymous fn/4 in Mix.Task.recur/1
    (elixir) lib/enum.ex:1940: Enum."-reduce/3-lists^foldl/2-0-"/3
    (mix) lib/mix/task.ex:430: Mix.Task.recur/1
    (mix) lib/mix/project_stack.ex:208: Mix.ProjectStack.recur/1
    lib/mix/tasks.ex:54: Mix.Tasks.Coveralls.do_run/2
    (mix) lib/mix/task.ex:331: Mix.Task.run_task/3
    (mix) lib/mix/cli.ex:79: Mix.CLI.run_task/2

[coveralls]

Coverage decreased (-0.05%) to 89.778% when pulling 8c55618 on diogommartins:fix/mocked-module-handling into 5f19e75 on parroty:master.

[diogommartins]

Hey, @parroty sorry but I was unable to figure out with I lowered the coverage on my PR. Could you help me out?

[parroty]

Thanks. I think the coverage change is just within the margin of error.