Enforce Go style rules across codebase

Apply five style rules: convert iota string enums to typed
string constants, replace errors.As with errors.AsType,
merge three-group imports into two groups, fix multiline
parameter/argument formatting, and replace fmt.Sprintf URL
construction with net/url.

Signed-off-by: Émile Ré <emile@probo.com>

pkg/agent/tools/security/spf.go

@@ -77,10 +77,12 @@ func CheckSPFTool() agent.Tool {
 				},
 			)
 			if err != nil {
-				return agent.ResultJSON(spfResult{
-					Found:       false,
-					ErrorDetail: fmt.Sprintf("cannot lookup SPF record: %s", err),
-				}), nil
+				return agent.ResultJSON(
+					spfResult{
+						Found:       false,
+						ErrorDetail: fmt.Sprintf("cannot lookup SPF record: %s", err),
+					},
+				), nil
 			}
 
 			var spfRecords []string
@@ -100,21 +102,25 @@ func CheckSPFTool() agent.Tool {
 			}
 
 			if len(spfRecords) > 1 {
-				return agent.ResultJSON(spfResult{
-					Found:       true,
-					ErrorDetail: fmt.Sprintf("multiple SPF records found (%d); this is an invalid configuration per RFC 7208", len(spfRecords)),
-				}), nil
+				return agent.ResultJSON(
+					spfResult{
+						Found:       true,
+						ErrorDetail: fmt.Sprintf("multiple SPF records found (%d); this is an invalid configuration per RFC 7208", len(spfRecords)),
+					},
+				), nil
 			}
 
 			if len(spfRecords) == 1 {
 				record := spfRecords[0]
 
-				return agent.ResultJSON(spfResult{
-					Found:      true,
-					RawRecord:  record,
-					Policy:     parseSPFPolicy(record),
-					Mechanisms: record,
-				}), nil
+				return agent.ResultJSON(
+					spfResult{
+						Found:      true,
+						RawRecord:  record,
+						Policy:     parseSPFPolicy(record),
+						Mechanisms: record,
+					},
+				), nil
 			}
 
 			return agent.ResultJSON(spfResult{Found: false}), nil