feat: instrument source files in parallel

[RomainMuller]

Instrumentation of source files can be performed in parallel, as the AST transformations do not span across files. Once the files are parsed & type checked, using a distinct decorator per file enables parallel processing & more efficient post-processing (fewer items in the decorator maps, none of which irrelevant to the current AST being post-processed).

This would improve the performance of builds, and a local profile showed a 70% improvement on the specific package I was playing with.

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Jira: APPSEC-54998

[codecov]

Codecov Report

Attention: Patch coverage is 70.21277% with 14 lines in your changes missing coverage. Please review.

Project coverage is 73.83%. Comparing base (6f519f1) to head (383872e).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/injector/injector.go	66.66%	10 Missing and 2 partials ⚠️
internal/injector/aspect/context/node.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #302      +/-   ##
==========================================
+ Coverage   73.77%   73.83%   +0.05%     
==========================================
  Files         145      145              
  Lines        7958     7972      +14     
==========================================
+ Hits         5871     5886      +15     
+ Misses       1658     1657       -1     
  Partials      429      429              

Components	Coverage Δ
Generators	76.69% <ø> (ø)
Instruments	88.05% <ø> (ø)
Go Driver	72.81% <ø> (ø)
Toolexec Driver	70.88% <ø> (ø)
Aspects	71.95% <0.00%> (ø)
Injector	73.42% <65.85%> (+<0.01%)	⬆️
Job Server	63.72% <100.00%> (+0.52%)	⬆️
Integration Test Suite	87.80% <ø> (ø)
Other	73.83% <70.21%> (+0.05%)	⬆️

Files with missing lines	Coverage Δ
internal/injector/restorer.go	75.00% <100.00%> (+0.71%)	⬆️
internal/jobserver/pkgs/resolve.go	89.38% <100.00%> (+4.95%)	⬆️
internal/injector/aspect/context/node.go	72.00% <0.00%> (ø)
internal/injector/injector.go	72.30% <66.66%> (+0.08%)	⬆️