Include completions for items from the same file; Clean up completions generation for callables

[sezna]

This PR closes #1862.

I had previously touched this function in #1820. I did my best to port it and maintain existing functionality, but there was one conditional that I messed up, and it was around callables originating from the same file.

This PR fixes the bug that caused. But also, this PR tries to simplify callable_decl_to_completion_item so this is less likely to happen in the future.

[github-actions]

Benchmark for 770b5c1

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Test	Base	PR	%
Array append evaluation	336.6±2.48µs	336.2±2.01µs	-0.12%
Array literal evaluation	186.5±3.33µs	186.2±1.00µs	-0.16%
Array update evaluation	416.5±2.79µs	416.0±2.85µs	-0.12%
Core + Standard library compilation	22.6±0.71ms	23.0±0.64ms	+1.77%
Deutsch-Jozsa evaluation	5.1±0.07ms	5.0±0.05ms	-1.96%
Large file parity evaluation	34.3±0.10ms	34.3±0.57ms	0.00%
Large input file compilation	13.9±0.35ms	14.4±0.37ms	+3.60%
Large input file compilation (interpreter)	52.8±1.59ms	53.1±2.06ms	+0.57%
Large nested iteration	32.6±0.16ms	32.7±0.19ms	+0.31%
Perform Runtime Capabilities Analysis (RCA) on Deutsch-Jozsa sample	1568.4±41.64µs	1565.2±75.09µs	-0.20%
Perform Runtime Capabilities Analysis (RCA) on large file sample	7.9±0.10ms	7.9±0.12ms	0.00%
Perform Runtime Capabilities Analysis (RCA) on teleport sample	1435.9±56.73µs	1422.4±40.27µs	-0.94%
Perform Runtime Capabilities Analysis (RCA) on the core and std libraries	27.9±0.24ms	27.9±0.40ms	0.00%
Teleport evaluation	94.6±3.58µs	94.1±4.11µs	-0.53%

[github-actions]

Benchmark for ddc3954

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Test	Base	PR	%
Array append evaluation	337.7±1.90µs	339.4±16.93µs	+0.50%
Array literal evaluation	169.2±0.89µs	186.1±0.83µs	+9.99%
Array update evaluation	416.4±5.00µs	415.8±3.85µs	-0.14%
Core + Standard library compilation	25.3±1.74ms	25.2±1.61ms	-0.40%
Deutsch-Jozsa evaluation	5.0±0.07ms	5.0±0.06ms	0.00%
Large file parity evaluation	34.8±0.23ms	34.7±0.31ms	-0.29%
Large input file compilation	15.3±0.86ms	13.8±0.43ms	-9.80%
Large input file compilation (interpreter)	59.0±3.15ms	55.2±3.03ms	-6.44%
Large nested iteration	33.0±1.50ms	32.9±0.49ms	-0.30%
Perform Runtime Capabilities Analysis (RCA) on Deutsch-Jozsa sample	1616.1±79.46µs	1629.6±184.19µs	+0.84%
Perform Runtime Capabilities Analysis (RCA) on large file sample	8.6±0.33ms	8.3±0.22ms	-3.49%
Perform Runtime Capabilities Analysis (RCA) on teleport sample	1486.0±155.61µs	1477.7±138.04µs	-0.56%
Perform Runtime Capabilities Analysis (RCA) on the core and std libraries	28.8±0.60ms	28.7±0.47ms	-0.35%
Teleport evaluation	94.8±3.64µs	94.2±3.72µs	-0.63%