Add file matches to detection result

[adracus]

This adds file matches to the detection result, changing the API as minimally as possible from map[string]float32 to map[string]api.Match where api.Match is

type Match struct {
    Confidence float32
    Files map[string]float32
}

I had to add an api package in order not to have any conversions or cyclic dependencies between the internal and licensedb package.

Fixes #55

[vmarkovtsev]

Wow this PR is so awesome, thank you very much for such polished code!

A few import formats and we are done.

[adracus]

Hi, thanks for the feedback! I've addressed your comments, hope that's okay!

[vmarkovtsev]

I will release v3 tomorrow.