Speedup dependency resolver by pre-cutting unsolvable branches

[cmaglie]

This change speed up cases where a lot of dependencies are present but one is missing:

A
|__ B (1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4)
|__ C (1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4)
|__ D (1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4)
|__ E (1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4)
|__ F (1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4)
|__ G (1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4)
|__ H (1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4)
\__ I (dependency not available)

In this case the last dependency I can not be resolved, but it is checked as last. This means that the backtracking algorithm will check all the other branches first with exponential time complexity (in the case shown above it must check 5^7 combinations before giving up).

[cmaglie]

See arduino/arduino-cli#2135 for a real-world use case

[codecov]

Codecov Report

Merging #14 (1e9020d) into master (9dd7f57) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #14   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          804       815   +11     
=========================================
+ Hits           804       815   +11     

Impacted Files	Coverage Δ
resolver.go	100.00% <100.00%> (ø)

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