meson: adjust nlohmann-json dependency

- Simplify nlohmann-json dependency in meson.build.
- Use consistent dependency variable names (based on Meson style).
- Align wrap file name with meson wrapdb

meson: adjust nlohmann-json dependency

- Simplify nlohmann-json dependency in meson.build.
- Use consistent dependency variable names (based on Meson style).
- Align wrap file name with meson wrapdb choice for the project.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I2e893fd8c300b36a16d704f2002bb966a6e52ce4

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