Update go-license-detector to v3

[johanbrandhorst]

go-license-detector v2 was depending on an old version of
gopkg.in/src-d/go-git, and unfortunately the latest version
broke compatibility unexpectedly. This made golicense
unnecessarily difficult to build in repositories which otherwise
used gopkg.in/src-d/go-git. Upgrading to v3 removes these problems.
See src-d/go-git#1262 for full context.

It also allows us to remove a whole lot of packages from go.mod
in general.

[mc-a]

For anyone looking how to use this fixed version in go1.16 or later, I got the following to work:

echo 'replace github.com/mitchellh/golicense => github.com/johanbrandhorst/golicense f78e380eb4ebd9f0c69f42932d5ff4c10f5e3a76' >> go.mod
go get github.com/mitchellh/golicense@v0.2.1-0.20191219125333-f78e380eb4eb

---

The following by itself doesn't work:

$ go get github.com/mitchellh/golicense@f78e380eb4ebd9f0c69f42932d5ff4c10f5e3a76
go get: github.com/mitchellh/golicense@f78e380eb4ebd9f0c69f42932d5ff4c10f5e3a76: invalid version: unknown revision f78e380eb4ebd9f0c69f42932d5ff4c10f5e3a76
$

$ go get github.com/johanbrandhorst/golicense@f78e380eb4ebd9f0c69f42932d5ff4c10f5e3a76
go get: github.com/johanbrandhorst/golicense@none updating to
        github.com/johanbrandhorst/golicense@v0.2.1-0.20191219125333-f78e380eb4eb: parsing go.mod:
        module declares its path as: github.com/mitchellh/golicense
                but was required as: github.com/johanbrandhorst/golicense
$

---

The error is reproducible by running go get github.com/google/go-licenses@v1.4.0 before go get github.com/mitchellh/golicense in a fresh Docker image — otherwise, this workaround isn't required:

bash-4.2$ go get github.com/google/go-licenses@v1.4.0
go get: added github.com/google/go-licenses v1.4.0
bash-4.2$ go get github.com/mitchellh/golicense
# gopkg.in/src-d/go-license-detector.v2/licensedb/filer
/go-mod-cache/gopkg.in/src-d/go-license-detector.v2@v2.0.0-20180510072912-da552ecf050b/licensedb/filer/filer.go:212:19: assignment mismatch: 2 variables but filesystem.NewStorage returns 1 values
/go-mod-cache/gopkg.in/src-d/go-license-detector.v2@v2.0.0-20180510072912-da552ecf050b/licensedb/filer/filer.go:212:43: not enough arguments in call to filesystem.NewStorage
        have (sivafs.SivaFS)
        want (billy.Filesystem, cache.Object)
bash-4.2$