feat: never build twice

[RomainMuller]

Adds a new jobserver message to track ongoing compilation tasks, so that duplicates can be completely avoided by re-using the first build task's outcome. This allows de-duplicating builds even if an injected package is already present in the natural dependency closure of the package being built.

Fixes #430

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 70 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@c163726). Learn more about missing BASE report.
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/toolexec/proxy/compile.go	68.25%	10 Missing and 10 partials ⚠️
internal/jobserver/nbt/nbt.go	82.65%	11 Missing and 6 partials ⚠️
internal/cmd/toolexec.go	44.82%	8 Missing and 8 partials ⚠️
internal/files/files.go	80.00%	2 Missing and 2 partials ⚠️
internal/jobserver/server.go	63.63%	2 Missing and 2 partials ⚠️
internal/toolexec/aspect/oncompile.go	55.55%	2 Missing and 2 partials ⚠️
internal/goflags/flags.go	92.30%	1 Missing ⚠️
internal/jobserver/buildid/version.go	66.66%	1 Missing ⚠️
internal/jobserver/pkgs/resolve.go	75.00%	1 Missing ⚠️
internal/toolexec/aspect/resolve.go	50.00%	1 Missing ⚠️
... and 1 more

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #490   +/-   ##
=======================================
  Coverage        ?   65.00%           
=======================================
  Files           ?      188           
  Lines           ?    11582           
  Branches        ?        0           
=======================================
  Hits            ?     7529           
  Misses          ?     3477           
  Partials        ?      576           

Components	Coverage Δ
Generators	76.15% <ø> (?)
Instruments	73.41% <ø> (?)
Go Driver	78.29% <92.30%> (?)
Toolexec Driver	73.72% <71.26%> (?)
Aspects	80.10% <ø> (?)
Injector	78.17% <ø> (?)
Job Server	70.68% <81.60%> (?)
Integration Test Suite	57.58% <ø> (?)
Other	65.00% <75.00%> (?)

Files with missing lines	Coverage Δ
internal/cmd/server.go	18.01% <100.00%> (ø)
internal/jobserver/client/client.go	57.89% <100.00%> (ø)
internal/jobserver/client/env.go	71.21% <100.00%> (ø)
internal/jobserver/common/handler.go	50.00% <100.00%> (ø)
internal/toolexec/proxy/compile.flags.go	100.00% <100.00%> (ø)
internal/toolexec/proxy/link.go	57.14% <100.00%> (ø)
internal/toolexec/proxy/parse.go	78.94% <100.00%> (ø)
internal/toolexec/proxy/proxy.go	74.13% <100.00%> (ø)
internal/toolexec/version.go	50.00% <100.00%> (ø)
internal/goflags/flags.go	79.37% <92.30%> (ø)
... and 10 more