[Builder] Add package option to support generating library (non-main) Go packages and improve build logging

[tomatopunk]

Description

This PR adds a new package option to the builder configuration, allowing the generated OpenTelemetry Collector sources to use a custom package name instead of main. When the package is set to a value other than main, the build process is updated to recognize that the output is intended as a Go library for import by other Go modules, rather than an executable binary.

Logging is improved to provide clear messaging about this build mode.

Link to tracking issue

Fixes #

Testing

Documentation

Added documentation for the new package option in the example builder configuration and usage docs.
Clarified build output expectations when using non-main packages.

[dmathieu]

This should be tested.

[tomatopunk]

This should be tested.

Added two improvements in the unit tests:

1. enhanced config_test to validate the default config
2. after running Generate, added a check to ensure the package statement in each generated Go file matches Distribution.Package

[dmathieu]

I'm a bit dubious about parsing Go code in a test to ensure it compiles. That seems a bit brittle.
How about doing something similar to the test folder that actually generates a collector and ensure it can run, to generate a library collector and ensure golangci-lint passes?

[tomatopunk]

There is a problem: the generated library collector cannot be run directly. Instead, it should be used as a referenced code library. Therefore, I only check whether the generated code meets our expectations, rather than whether it runs independently.

[tomatopunk]

The library collector can be compiled, but it cannot run because it lacks a main package. That’s why, at https://github.com/open-telemetry/opentelemetry-collector/pull/13296/files#diff-117127e4cf9f6587102dc92a7240d1e2c00944ee47ce23e02731f3a41466bdb1R109-R112 in the compline function, I exit if the package is not main and do not proceed with compilation.

In my program, I manually create an export.go file, which defines an exportable function that calls the main method. This allows the upper layer to import and use it.

[bogdandrutu]

Can you create the main func in your test directory and call the library?

[codecov]

Codecov Report

Attention: Patch coverage is 37.50000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 91.60%. Comparing base (18b3b57) to head (63e02a6).

Files with missing lines	Patch %	Lines
cmd/builder/internal/builder/main.go	28.57%	3 Missing and 2 partials ⚠️

❌ Your patch check has failed because the patch coverage (37.50%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13296      +/-   ##
==========================================
- Coverage   91.64%   91.60%   -0.05%     
==========================================
  Files         522      522              
  Lines       29183    29189       +6     
==========================================
- Hits        26746    26739       -7     
- Misses       1920     1929       +9     
- Partials      517      521       +4     

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