fix: respect to $Devel::Cover::Silent

The issue:
without silent
```
Devel::Cover: Warning: can't open blib/lib/Net/SSLeay.pm for MD5 digest: No such file or directory
Devel::Cover: Can't find file "blib/lib/Net/SSLeay.pm" (blib/lib/Net/SSLeay.pm (autosplit into blib/lib/auto/Net/SSLeay/randomize.al)): ignored.
```
with '-silent,1' still
```
Devel::Cover: Warning: can't open blib/lib/Net/SSLeay.pm for MD5 digest: No such file or directory
```
But I do not want to see this :)

BTW the problem may be deeper - do not find correct some files (?)

P.S. Thanks for great module :)