traverse vanity urls for repository location for license url

[KnicKnic]

I ran against github.com/google/go-licenses and noticed that vanity urls weren't traversed for their corresponding github url.
see below for - cannot determine URL for "gopkg.in/warnings.v0" package
curl.exe https://gopkg.in/warnings.v0?go-get=1
yeilds
<meta name="go-source" content="gopkg.in/warnings.v0 _ https://github.com/go-warnings/warnings/tree/v0.1.2{/dir} https://github.com/go-warnings/warnings/blob/v0.1.2{/dir}/{file}#L{line}">

PS C:\Users\nmaliwa\go\src\github.com\google\go-licenses> .\go-licenses.exe csv .
E0706 20:48:36.424458   30424 csv.go:87] Error discovering URL for "C:\\Users\\nmaliwa\\go\\pkg\\mod\\gopkg.in\\warnings.v0@v0.1.2\\LICENSE":
- cannot determine URL for "gopkg.in/warnings.v0" package
E0706 20:48:36.576080   30424 csv.go:87] Error discovering URL for "C:\\Users\\nmaliwa\\go\\pkg\\mod\\golang.org\\x\\tools@v0.0.0-20191118222007-07fc4c7f2b98\\LICENSE":
- unsupported package host "golang.org" for "golang.org/x/tools"
E0706 20:48:36.601091   30424 csv.go:87] Error discovering URL for "C:\\Users\\nmaliwa\\go\\pkg\\mod\\gopkg.in\\src-d\\go-git.v4@v4.13.1\\LICENSE":
- unsupported package host "gopkg.in" for "gopkg.in/src-d/go-git.v4"
E0706 20:48:36.728275   30424 csv.go:87] Error discovering URL for "C:\\Users\\nmaliwa\\go\\pkg\\mod\\golang.org\\x\\crypto@v0.0.0-20191117063200-497ca9f6d64f\\LICENSE":
- unsupported package host "golang.org" for "golang.org/x/crypto"
E0706 20:48:36.741540   30424 csv.go:87] Error discovering URL for "C:\\Users\\nmaliwa\\go\\pkg\\mod\\gopkg.in\\src-d\\go-billy.v4@v4.3.2\\LICENSE":
- unsupported package host "gopkg.in" for "gopkg.in/src-d/go-billy.v4"
E0706 20:48:36.813157   30424 csv.go:87] Error discovering URL for "C:\\Users\\nmaliwa\\go\\pkg\\mod\\golang.org\\x\\sys@v0.0.0-20191119060738-e882bf8e40c2\\LICENSE":
- unsupported package host "golang.org" for "golang.org/x/sys/windows"
E0706 20:48:36.827165   30424 csv.go:87] Error discovering URL for "C:\\Users\\nmaliwa\\go\\pkg\\mod\\golang.org\\x\\net@v0.0.0-20191119073136-fc4aabc6c914\\LICENSE":
- unsupported package host "golang.org" for "golang.org/x/net"
github.com/google/licenseclassifier/stringclassifier,https://github.com/google/licenseclassifier/blob/master/stringclassifier/LICENSE,Apache-2.0        
github.com/sergi/go-diff/diffmatchpatch,https://github.com/sergi/go-diff/blob/master/diffmatchpatch/LICENSE,MIT
gopkg.in/warnings.v0,Unknown,BSD-2-Clause
github.com/otiai10/copy,https://github.com/otiai10/copy/blob/master/LICENSE,MIT
github.com/golang/glog,https://github.com/golang/glog/blob/master/LICENSE,Apache-2.0
golang.org/x/tools,Unknown,BSD-3-Clause
gopkg.in/src-d/go-git.v4,Unknown,Apache-2.0
golang.org/x/crypto,Unknown,BSD-3-Clause
gopkg.in/src-d/go-billy.v4,Unknown,Apache-2.0
golang.org/x/sys/windows,Unknown,BSD-3-Clause
golang.org/x/net,Unknown,BSD-3-Clause
github.com/mitchellh/go-homedir,https://github.com/mitchellh/go-homedir/blob/master/LICENSE,MIT
github.com/spf13/cobra,https://github.com/spf13/cobra/blob/master/LICENSE.txt,Apache-2.0
github.com/inconshreveable/mousetrap,https://github.com/inconshreveable/mousetrap/blob/master/LICENSE,Apache-2.0
github.com/google/go-licenses,https://github.com/google/go-licenses/blob/master/LICENSE,Apache-2.0
github.com/google/licenseclassifier,https://github.com/google/licenseclassifier/blob/master/LICENSE,Apache-2.0
github.com/spf13/pflag,https://github.com/spf13/pflag/blob/master/LICENSE,BSD-3-Clause
github.com/src-d/gcfg,https://github.com/src-d/gcfg/blob/master/LICENSE,BSD-3-Clause
github.com/jbenet/go-context/io,https://github.com/jbenet/go-context/blob/master/io/LICENSE,MIT
github.com/emirpasic/gods,https://github.com/emirpasic/gods/blob/master/LICENSE,BSD-2-Clause
github.com/kevinburke/ssh_config,https://github.com/kevinburke/ssh_config/blob/master/LICENSE,MIT
github.com/xanzy/ssh-agent,https://github.com/xanzy/ssh-agent/blob/master/LICENSE,Apache-2.0

[Bobgy]

Fixed in #94