feat: mockgen flag --build_constraint to add //go:build directives (#191)

Resolves #190

Note that `//go:generate` has unusual handling of flags with spaces so
the quotes have to include the flag name
([example](https://github.com/ARR4N/mock/blob/36b5e87796c7f82be7275e40bd54965c8605875c/mockgen/internal/tests/build_constraint/input.go#L3))
for complex constraints. This revealed a bug in
`--write_generate_directive`, which just prints space-delimited
`os.Args`.

The workaround of using `--copyright_file` is neither viable (it adds a
space between `//` and `go:build`) nor good practice as overloading
functionality can result in bugs due to unanticipated usage (e.g. the
aforementioned space).

Author: ARR4N

mockgen/internal/tests/build_constraint/input.go

@@ -0,0 +1,5 @@
+package empty_interface
+
+//go:generate mockgen -package empty_interface -destination mock.go -source input.go "-build_constraint=(linux && 386) || (darwin && !cgo) || usertag"
+
+type Empty interface{}

mockgen/internal/tests/build_constraint/mock.go

@@ -64,6 +64,7 @@ var (
 	writeSourceComment     = flag.Bool("write_source_comment", true, "Writes original file (source mode) or interface names (reflect mode) comment if true.")
 	writeGenerateDirective = flag.Bool("write_generate_directive", false, "Add //go:generate directive to regenerate the mock")
 	copyrightFile          = flag.String("copyright_file", "", "Copyright file used to add copyright header")
+	buildConstraint        = flag.String("build_constraint", "", "If non-empty, added as //go:build <constraint>")
 	typed                  = flag.Bool("typed", false, "Generate Type-safe 'Return', 'Do', 'DoAndReturn' function")
 	imports                = flag.String("imports", "", "(source mode) Comma-separated name=path pairs of explicit imports to use.")
 	auxFiles               = flag.String("aux_files", "", "(source mode) Comma-separated pkg=path pairs of auxiliary Go source files.")
@@ -143,7 +144,9 @@ func main() {
 		}
 	}
 
-	g := new(generator)
+	g := &generator{
+		buildConstraint: *buildConstraint,
+	}
 	if *source != "" {
 		g.filename = *source
 	} else {
@@ -251,6 +254,7 @@ type generator struct {
 	destination               string            // may be empty
 	srcPackage, srcInterfaces string            // may be empty
 	copyrightHeader           string
+	buildConstraint           string // may be empty
 
 	packageMap map[string]string // map from import path to package name
 }
@@ -306,6 +310,12 @@ func (g *generator) Generate(pkg *model.Package, outputPkgName string, outputPac
 		g.p("")
 	}
 
+	if g.buildConstraint != "" {
+		g.p("//go:build %s", g.buildConstraint)
+		// https://pkg.go.dev/cmd/go#hdr-Build_constraints:~:text=a%20build%20constraint%20should%20be%20followed%20by%20a%20blank%20line
+		g.p("")
+	}
+
 	g.p("// Code generated by MockGen. DO NOT EDIT.")
 	if *writeSourceComment {
 		if g.filename != "" {