refactor: redesign how we import assets (#260)

* fix(pkg.go.dev): import a subpackage containing the assets

We're trying to fix this issue that pkg.go.dev does not build.

Thanks to @hellais for this very neat idea! Let's keep our
fingers crossed and see whether it fixes!

* feat: use embedded geoip databases

Closes https://github.com/ooni/probe/issues/1372.

Work done as part of https://github.com/ooni/probe/issues/1369.

* fix(assetsx): add tests

* feat: simplify and just vendor uncompressed DBs

* remove tests that seems not necessary anymore

* fix: run go mod tidy

* Address https://github.com/ooni/probe-cli/pull/260/files#r605181364

* rewrite a test in a better way

* fix: gently cleanup the legacy assetsdir

Do not remove the whole directory with brute force. Just zap the
files whose name we know. Then attempt to delete the legacy directory
as well. If not empty, just fail. This is fine because it means the
user has stored other files inside the directory.

* fix: create .miniooni if missing

go.sum

@@ -335,6 +335,8 @@ github.com/onsi/gomega v1.5.0/go.mod h1:ex+gbHU/CVuBBDIJjb2X0qEXbFg53c61hWP/1Cpa
 github.com/onsi/gomega v1.7.1/go.mod h1:XdKZgCCFLUoM/7CFJVPcG8C1xQ1AJ0vpAezJrB7JYyY=
 github.com/onsi/gomega v1.10.1 h1:o0+MgICZLuZ7xjH7Vx6zS/zcu93/BEp1VwkIW1mEXCE=
 github.com/onsi/gomega v1.10.1/go.mod h1:iN09h71vgCQne3DLsj+A5owkum+a2tYe+TOCB1ybHNo=
+github.com/ooni/probe-assets v0.0.0-20210401100648-90ed7b6dff90 h1:G7urihGBD88p5iU1bg7cFAqX/38qIPZH03wCTmyUAXI=
+github.com/ooni/probe-assets v0.0.0-20210401100648-90ed7b6dff90/go.mod h1:N0PyNM3aadlYDDCFXAPzs54HC54+MZA/4/xnCtd9EAo=
 github.com/ooni/psiphon v0.6.0 h1:TWXFSlXBOFUwfGblLKaLXSoI7UL6dreoPXHW2uQTHlc=
 github.com/ooni/psiphon v0.6.0/go.mod h1:i1v6JweJtxDKaI0i1aEw2/Fr/CUi5BoQ75GYz5KmKwU=
 github.com/op/go-logging v0.0.0-20160315200505-970db520ece7/go.mod h1:HzydrMdWErDVzsI23lYNej1Htcns9BCg93Dk0bBINWk=