refactor: extract shared code for linting if-else chains (#821)

* refactor: extract shared code for linting if-else chains

The rules "early-return", "indent-error-flow" and
"superfluous-else" have a similar structure. This
moves the common logic for classifying if-else chains
to a common package.

A few side benefits:
- "early-return" now handles os.Exit/log.Panicf/etc
- "superfluous-else" now handles (builtin) panic
- "superfluous-else" and "indent-error-flow" now handle if/else
  chains with 2+ "if" branches

* internal/ifelse: style fixes, renames, spelling

internal/ifelse/branch.go

@@ -0,0 +1,68 @@
+package ifelse
+
+import (
+	"fmt"
+	"go/ast"
+	"go/token"
+)
+
+// Branch contains information about a branch within an if-else chain.
+type Branch struct {
+	BranchKind
+	Call // The function called at the end for kind Panic or Exit.
+}
+
+// BlockBranch gets the Branch of an ast.BlockStmt.
+func BlockBranch(block *ast.BlockStmt) Branch {
+	blockLen := len(block.List)
+	if blockLen == 0 {
+		return Branch{BranchKind: Empty}
+	}
+
+	switch stmt := block.List[blockLen-1].(type) {
+	case *ast.ReturnStmt:
+		return Branch{BranchKind: Return}
+	case *ast.BlockStmt:
+		return BlockBranch(stmt)
+	case *ast.BranchStmt:
+		switch stmt.Tok {
+		case token.BREAK:
+			return Branch{BranchKind: Break}
+		case token.CONTINUE:
+			return Branch{BranchKind: Continue}
+		case token.GOTO:
+			return Branch{BranchKind: Goto}
+		}
+	case *ast.ExprStmt:
+		fn, ok := ExprCall(stmt)
+		if !ok {
+			break
+		}
+		kind, ok := DeviatingFuncs[fn]
+		if ok {
+			return Branch{BranchKind: kind, Call: fn}
+		}
+	}
+
+	return Branch{BranchKind: Regular}
+}
+
+// String returns a brief string representation
+func (b Branch) String() string {
+	switch b.BranchKind {
+	case Panic, Exit:
+		return fmt.Sprintf("... %v()", b.Call)
+	default:
+		return b.BranchKind.String()
+	}
+}
+
+// LongString returns a longer form string representation
+func (b Branch) LongString() string {
+	switch b.BranchKind {
+	case Panic, Exit:
+		return fmt.Sprintf("call to %v function", b.Call)
+	default:
+		return b.BranchKind.LongString()
+	}
+}