Support pinned versions, e.g., "core@1 = 1.0.0"

[jneums]

Root cause: The mops canister expects the base package name "core" but receives "core@1".

Fix: Extract base name (before @) when querying mops API, while preserving full name in lock file and moc args for imports like mo:core@1/Iter.

Solution implemented:

• Added dep_base() helper: "core@1" → "core"
• Updated 4 mops service call sites to use base name
• Kept full name everywhere else (matches TypeScript implementation)

Tested with "core@1" = "1.0.0" - now downloads and compiles successfully.

PR: #12