Clean up DiscoveryService

[caendesilva]

Main changes:

• Throw exception instead of returning false when supplied invalid class

[codecov]

Codecov Report

Merging #305 (f10daa2) into master (5697712) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master      #305   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity       825       825           
===========================================
  Files             99        99           
  Lines           2066      2065    -1     
===========================================
- Hits            2066      2065    -1     

Impacted Files	Coverage Δ
packages/framework/src/Foundation/Filesystem.php	100.00% <100.00%> (ø)
...ckages/framework/src/Services/DiscoveryService.php	100.00% <100.00%> (ø)

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