fix: OnCompileMain needs to register new link.deps

[RomainMuller]

The OnCompile hook cannot necessarily resolve the transitive dependencies of all link-time dependencies, as those are normally used to avoid creating dependency cycles.

The OnCompileMain hook however MUST resolve the transitive dependencies of everything, as it needs to introduce imports of all synthetic link-time dependencies so that they are correctly registered (their init functions get scheduled, and they get presented to the linker). This was previously not necessarily done, and could result in link-time failures.

---

This involved re-working the way the link.deps file is added to the _pkg_.a files so that both the OnCompile and OnCompileMain functions contribute to the same closure and it gets written & registered exactly once. As a result Command.OnClose was removed (no longer used).

[codecov]

Codecov Report

Attention: Patch coverage is 62.12121% with 25 lines in your changes missing coverage. Please review.

Project coverage is 62.41%. Comparing base (bf4e88f) to head (b3230da).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/toolexec/proxy/compile.go	41.66%	14 Missing and 7 partials ⚠️
internal/toolexec/aspect/oncompile-main.go	84.21%	2 Missing and 1 partial ⚠️
internal/toolexec/aspect/linkdeps/linkdeps.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #501      +/-   ##
==========================================
- Coverage   65.11%   62.41%   -2.70%     
==========================================
  Files         186      186              
  Lines       11348    13327    +1979     
==========================================
+ Hits         7389     8318     +929     
- Misses       3395     4464    +1069     
+ Partials      564      545      -19     

Components	Coverage Δ
Generators	74.13% <ø> (-2.03%)	⬇️
Instruments	66.29% <ø> (-7.13%)	⬇️
Go Driver	72.20% <ø> (-5.00%)	⬇️
Toolexec Driver	70.23% <61.53%> (-3.75%)	⬇️
Aspects	76.20% <ø> (-3.90%)	⬇️
Injector	74.95% <ø> (-3.08%)	⬇️
Job Server	63.92% <ø> (-9.17%)	⬇️
Integration Test Suite	57.21% <ø> (-0.37%)	⬇️
Other	62.41% <62.12%> (-2.70%)	⬇️

Files with missing lines	Coverage Δ
internal/cmd/toolexec.go	54.54% <100.00%> (-2.13%)	⬇️
internal/toolexec/aspect/oncompile.go	72.30% <100.00%> (+0.17%)	⬆️
internal/toolexec/proxy/proxy.go	69.11% <100.00%> (-3.90%)	⬇️
internal/toolexec/aspect/linkdeps/linkdeps.go	63.56% <66.66%> (-1.49%)	⬇️
internal/toolexec/aspect/oncompile-main.go	63.95% <84.21%> (-1.44%)	⬇️
internal/toolexec/proxy/compile.go	61.45% <41.66%> (-16.81%)	⬇️

... and 160 files with indirect coverage changes