Leftover of #1354: module-based completion, tests

[ice1000]

Missing:

• Provide completion information by the identifier at the given position, for example: provide all symbols in Foo::Nat if Foo::Nat:_, provide all symbols in Foo starts with Nat if Foo::Nat_.
• Testing about completion (especially aya-vscode)

Originally posted by @ice1000 in #1352 (comment)