Remove feature flags and the fallback command

See merge request gitlab-org/gitlab-shell!336

Build against go1.13

See merge request gitlab-org/gitlab-shell!337

Release v10.0.0

See merge request gitlab-org/gitlab-shell!333

Remove gitlab-keys script

See merge request gitlab-org/gitlab-shell!329

Refactor "go build" path constants

See merge request gitlab-org/gitlab-shell!328

Release v9.4.2

See merge request gitlab-org/gitlab-shell!331

Repurpose bin/authorized_keys script

See merge request gitlab-org/gitlab-shell!330

It's been replaced by `gitlab-shell-authorized-keys-check` and
it's also calling the `/authorized_keys` endpoint via Ruby.

Since they have the same functionality as the new binary, better
to call that instead to make it SSOT for authorized keys requests.

Fix bin/compile in GDK

Closes gitlab-development-kit#585

See merge request gitlab-org/gitlab-shell!327

Release v9.4.0

See merge request gitlab-org/gitlab-shell!325

Use go mod

See merge request gitlab-org/gitlab-shell!323

Replace symlinks with actual binaries

Closes #206

See merge request gitlab-org/gitlab-shell!324

We had `gitlab-shell-authorized-keys-check` and
`gitlab-shell-authorized-principals-check` as symlinks to
`gitlab-shell` before.

We determine the `Command` and `CommandArgs` that we build based
on the `Name` of the `Executable`. We also use that to know which
fallback ruby executable should we fallback to. We use
`os.Executable()` to do that.

`os.Executable()` behaves differently depending on OS. It may
return the symlink or the target's name. That can result to a
buggy behavior.

The fix is to create binaries for each instead of using a symlink.
That way we don't need to rely on `os.Executable()` to get the name.
We pass the `Name` of the executable instead.

Implement AuthorizedPrincipals command

Closes #181

See merge request gitlab-org/gitlab-shell!322

Build this command when `Executable` name is
`gitlab-shell-authorized-principals-check`. Feature flag is the
same name.

Implement AuthorizedKeys command

See merge request gitlab-org/gitlab-shell!321

Build this command when `Executable` name is
`gitlab-shell-authorized-keys-check`. Feature flag is the same
name.

Support falling back to ruby version of checkers

See merge request gitlab-org/gitlab-shell!318

This struct is responsible for determining the name and
root dir of the executable.

The `RootDir` property will be used to find the config.

The `Name` property will be used to determine what `Command`
and `CommandArgs` to be built.

Other functions are still expecting for `CommandArgs` instead
of `Shell`. They should be expecting `commandargs.Shell` now
since it has been renamed.

`CommandArgs` has been renamed to `Shell`.

An interface has been added that includes `Executable()` and
`Arguments()` method. The `BaseArgs` implement this methods
and should be embeeded in each type.