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/rsa.go

@@ -40,7 +40,7 @@ func (w *WeakKeyStrength) Match(n ast.Node, c *gas.Context) (*gas.Issue, error)
 func NewWeakKeyStrength() (r gas.Rule, n ast.Node) {
 	bits := 2048
 	r = &WeakKeyStrength{
-		pattern: regexp.MustCompile(`^rsa.GenerateKey$`),
+		pattern: regexp.MustCompile(`^rsa\.GenerateKey$`),
 		bits:    bits,
 		MetaData: gas.MetaData{
 			Severity:   gas.Medium,