index: use ID based shard names only on multi-tenant instances

[keegancsmith]

We only want to use ID based shard names on multi-tenant instances.
Before this change we decided this based on if tenant enforcement was
turned on. However, we would like to always have tenant enforcment on at
Sourcegraph but not migrate all shards to the new naming scheme (just
yet).

This change is quite simple. However, it works since we never actually
read the filenames to extract tenant ID or repository ID. We only ever
use the data that is persisted inside of the shard to do the
enforcement.

The environment variable we read (WORKSPACES_API_URL) is the same one we
use in the Sourcegraph codebase to determine multi-tenant specific code.

Test Plan: Added a unit test. Will do a much larger manual E2E test
with the Sourcegraph project.

Closes https://linear.app/sourcegraph/issue/DINF-940/zoekt-enforcetenant-turned-on-works-for-non-workspace-instances

[keegancsmith]

This change is part of the following stack:

• index: decide between tenant and non-tenant shard name in one place #953
  • index: remove Options.{TenantID,RepoID} #954
    • index: inline tenant.SrcPrefix #955
      • index: use ID based shard names only on multi-tenant instances #956 ◀
        • tenant: unexport EnforceTenant #957

_{Change managed by git-spice.}