frontend: add build-time source filters

[cpuguy83]

Add a source-options context that can provide a YAML source filter
config at build time. The config currently supports global_excludes
which can be used to filter out paths from all sources.

[Copilot]

Pull request overview

This PR adds build-time source filtering support via a YAML config supplied through a build context, so package/source generation can exclude globally configured paths without changing the Dalec spec.

Changes:

• Adds SourceFilterConfig, build args, frontend loading, and source/generator filtering hooks.
• Propagates filter metadata into RPM spec generation and source docs.
• Adds unit/integration coverage and user documentation for source filter behavior.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
website/docs/sources.md	Documents build-time source filter configuration and Buildx usage.
test/source_test.go	Adds integration tests for debug source and gomod filtering.
targets/windows/handle_zip.go	Adjusts Windows binary build source options around filtering.
spec_test.go	Tests YAML decoding and emptiness checks for filter config.
source.go	Adds filter plumbing to source fetch options and source docs.
source_test.go	Adds unit tests for source filter LLB behavior.
source_inline.go	Adjusts inline directory include/exclude paths when renamed.
source_filter.go	Introduces source filter config, constants, and LLB filter helpers.
packaging/linux/rpm/template.go	Emits source filter documentation and pip source entries in RPM specs.
packaging/linux/rpm/template_test.go	Updates RPM source expectations and filter documentation tests.
packaging/linux/rpm/buildroot.go	Passes source filter config into RPM spec generation.
load.go	Marks source filter build args as known arguments.
generator_pip.go	Applies filters to generated pip dependency cache.
generator_gomod.go	Applies filters to generated gomod dependency cache.
generator_cargohome.go	Applies filters to generated Cargo dependency cache.
frontend/request.go	Loads source filter config from a build context.
frontend/gateway.go	Wires cached source filter loading into frontend source options.
frontend/debug/handle_sources.go	Passes progress options while preparing debug sources.

Comments suppressed due to low confidence (1)

packaging/linux/rpm/template.go:386

• Filter patterns are written directly into the generated RPM spec comment. If a pattern contains a newline, only the first line is prefixed with #, so the remaining text can become active spec content; comment each line (as Sources does when scanning src.Doc) or sanitize newlines before writing.

	}

	if len(keys) > 0 {
		b.WriteString("\n")

[Copilot]

Pull request overview

Copilot reviewed 19 out of 19 changed files in this pull request and generated 1 comment.

[MorrisLaw]

I think this might be a little too early in the flow.

Since this gets added in Source.fetchOptions, it looks like the filter applies any time we materialize a directory source, including sources that are only being used as inputs to generators. IIUC that means a filter could exclude something like go.sum, requirements.txt, or Cargo.lock before the generator has a chance to use it.

Since we also filter the generated cache later, maybe this should live closer to the final source-package/debug-source assembly instead of in the generic source path?

Could we add a regression test for this case just to make sure?