Fix incorrect regexp matches

There are some cases where the '.' character would also match any
character and could lead to incorrect results. For example the
regular expression -  `^ioutils.WriteFile$' would match
ioutils.WriteFile, but also ioutils_WriteFile.

Additionally made sure that all regexp were declared using raw
strings to avoid any unnecesary string escaping that potentially
make the regexp difficult to read.

rules/hardcoded_credentials.go

@@ -45,7 +45,7 @@ func (r *CredsAssign) Match(n ast.Node, c *gas.Context) (gi *gas.Issue, err erro
 
 func NewHardcodedCredentials() (r gas.Rule, n ast.Node) {
 	r = &CredsAssign{
-		pattern: regexp.MustCompile("(?i)passwd|pass|password|pwd|secret|token"),
+		pattern: regexp.MustCompile(`(?i)passwd|pass|password|pwd|secret|token`),
 		MetaData: gas.MetaData{
 			What:       "Potential hardcoded credentials",
 			Confidence: gas.Low,