Use DocumentUri converter directly in Merlin Call Compatible params

[voodoos]

It makes the intent clearer than relying on "unwrapping" a TextDocumentIdentifier.t (which is an object with a unique uri : DocumentUri.t field).

Ideally, we could change the spec of the request to better fit into the protocol to have a textDocument : TextDocumentIdentifier.t instead of a uri : DocumentUri.t. The same information is conveyed, but the former version is the one used across the protocol and most other custom queries.

We could transition by recognizing both fields until the clients are all updated ?

This is a follow-up to #1428.

cc @awilliambauer

[coveralls]

Pull Request Test Coverage Report for Build 4671

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.004%) to 22.171%

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Files with Coverage Reduction	New Missed Lines	%
ocaml-lsp-server/src/import.ml	1	34.69%

Totals
Change from base Build 4668:	-0.004%
Covered Lines:	5650
Relevant Lines:	25484

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💛 - Coveralls