Lint: lint and fix src/{os,reflect} (#4228)

* lint: expand to src/{os,reflect}, fix or suppress what it found

* internal/tools/tools.go: the tools idiom requires a build tag guard to avoid go test complaints

revive.toml

@@ -6,15 +6,19 @@ warningCode = 0
 
 # Enable these as we fix them
 [rule.blank-imports]
+  Exclude=["src/os/file_other.go"]
 [rule.context-as-argument]
 [rule.context-keys-type]
 [rule.dot-imports]
+  Exclude=["**/*_test.go"]
 [rule.error-return]
 [rule.error-strings]
 [rule.error-naming]
 [rule.exported]
+  Exclude=["src/reflect/*.go"]
 [rule.increment-decrement]
 [rule.var-naming]
+  Exclude=["src/os/*.go"]
 [rule.var-declaration]
 #[rule.package-comments]
 [rule.range]
@@ -27,4 +31,5 @@ warningCode = 0
 [rule.superfluous-else]
 #[rule.unused-parameter]
 [rule.unreachable-code]
+  Exclude=["src/reflect/visiblefields_test.go", "src/reflect/all_test.go"]
 #[rule.redefines-builtin-id]