WIP [builder] Support for --skip-new-go-module proof of concept

cmd/builder/README.md

@@ -145,3 +145,44 @@ then commit the code in a git repo. A CI can sync the code and execute
 ocb --skip-generate --skip-get-modules --config=config.yaml
 ```
 to only execute the compilation step.
+
+### Avoiding the use of a new go.mod file
+
+There is an additional option that controls one aspect of the build
+process, which specifically allows skipping the use of a new `go.mod`
+file.  When the `--skip-new-go-module` command-line flag is supplied,
+the build process issues a `go get` command for each component,
+relying on the Go toolchain to update the enclosing Go module
+appropriately.
+
+This command will avoid downgrading a dependency in the enclosing
+module, according to
+[`semver.Compare()`](https://pkg.go.dev/golang.org/x/mod/semver#Compare),
+and will instead issue a log indicating that the component was not
+upgraded.
+
+This mode cannot be used in conjunction with several features that
+control the generated `go.mod` file, including `replaces`, `excludes`,
+and the component `path` override.  For each of these features, users
+are expected to modify the enclosing `go.mod` directly.
+
+### Strict versioning checks
+
+There is an option that controls version strictness applied by the
+builder.  When the `--strict-versioning` command-line flag is used,
+the following additional checks apply to the finished `go.mod` file
+after getting all the components and tidying the result. 
+
+1. The `dist::otelcol_version` field in the build configuration must
+   match the core library version calculated by the Go toolchain,
+   considering all components.  A mismatch could happen, for example,
+   when one of the components depends on a newer release of the core
+   collector library.
+2. For each component in the build configuration, the version included
+   in the `gomod` module specifier must match the one calculated by
+   the Go toolchain, considering all components.  A mismatch could
+   happen, for example, when the enclosing Go module uses a newer
+   release of the core collector library.
+
+When `--skip-new-go-module` is used, these checks apply to the
+enclosing Go module; otherwise, they apply to the generated module.

cmd/builder/go.mod

@@ -31,6 +31,7 @@ require (
 	github.com/mitchellh/reflectwalk v1.0.2 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/rogpeppe/go-internal v1.10.0 // indirect
+	golang.org/x/mod v0.14.0 // indirect
 	golang.org/x/sys v0.14.0 // indirect
 	gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect

cmd/builder/internal/builder/config.go

@@ -23,12 +23,14 @@ var ErrInvalidGoMod = errors.New("invalid gomod specification for module")
 
 // Config holds the builder's configuration
 type Config struct {
-	Logger          *zap.Logger
-	SkipGenerate    bool   `mapstructure:"-"`
-	SkipCompilation bool   `mapstructure:"-"`
-	SkipGetModules  bool   `mapstructure:"-"`
-	LDFlags         string `mapstructure:"-"`
-	Verbose         bool   `mapstructure:"-"`
+	Logger           *zap.Logger
+	SkipGenerate     bool   `mapstructure:"-"`
+	SkipCompilation  bool   `mapstructure:"-"`
+	SkipGetModules   bool   `mapstructure:"-"`
+	SkipNewGoModule  bool   `mapstructure:"-"`
+	StrictVersioning bool   `mapstructure:"-"`
+	LDFlags          string `mapstructure:"-"`
+	Verbose          bool   `mapstructure:"-"`
 
 	Distribution Distribution `mapstructure:"dist"`
 	Exporters    []Module     `mapstructure:"exporters"`
@@ -87,11 +89,12 @@ func NewDefaultConfig() Config {
 // Validate checks whether the current configuration is valid
 func (c *Config) Validate() error {
 	return multierr.Combine(
-		validateModules(c.Extensions),
-		validateModules(c.Receivers),
-		validateModules(c.Exporters),
-		validateModules(c.Processors),
-		validateModules(c.Connectors),
+		c.validateModules(c.Extensions),
+		c.validateModules(c.Receivers),
+		c.validateModules(c.Exporters),
+		c.validateModules(c.Processors),
+		c.validateModules(c.Connectors),
+		c.validateFlags(),
 	)
 }
 
@@ -158,20 +161,35 @@ func (c *Config) ParseModules() error {
 	return nil
 }
 
-func validateModules(mods []Module) error {
+func (c *Config) validateFlags() error {
+	if c.SkipNewGoModule && (len(c.Replaces) != 0 || len(c.Excludes) != 0) {
+		return fmt.Errorf("cannot combine excludes or replaces with --skip-new-go-module; please modify the enclosing go.mod file directly, in this case")
+	}
+	return nil
+}
+
+func (c *Config) validateModules(mods []Module) error {
 	for _, mod := range mods {
 		if mod.GoMod == "" {
 			return fmt.Errorf("module %q: %w", mod.GoMod, ErrInvalidGoMod)
 		}
+		if mod.Path != "" && c.SkipNewGoModule {
+			return fmt.Errorf("cannot modify gomod path combined with --skip-new-go-module; please modify the enclosing go.mod file directly, in this case")
+		}
 	}
 	return nil
 }
 
 func parseModules(mods []Module) ([]Module, error) {
 	var parsedModules []Module
 	for _, mod := range mods {
+		module, _, found := strings.Cut(mod.GoMod, " ")
+		if !found {
+			return mods, fmt.Errorf("go.mod module specifier syntax is <module><space><version>: %q", mod.GoMod)
+		}
+
 		if mod.Import == "" {
-			mod.Import = strings.Split(mod.GoMod, " ")[0]
+			mod.Import = module
 		}
 
 		if mod.Name == "" {