Fix another scenario that was creating duplicate licenses

Commits 8e041bf and 7f40b96 attempted to prohibit creating duplicate
LicenseAlias records.  This commit prevents

```
   license_finder dependencies add MIT first_dep 1.1
   license_finder dependencies add MIT second_dep 2.2
```

from creating many MIT LicenseAliases.
Remove dead code
make public api of PossibleLicenseFiles explicit
Merge pull request #72 from mainej/prevent-duplicate-aliases

Prevent duplicate licenses