interp: allow redeclaration of imports

This PR adds an interpreter option, `AllowRedeclaration`. If this option is set, `(*Interpreter).Eval` will allow package imports to be redeclared. That is, no error will be raised and the package symbol will be overwritten.

I would like to use Yaegi to power a Go notebook (VSCode extension), somewhat like Jupyter. A notebook can have multiple Go 'cells' which can be evaluated (using Yaegi). As much as is possible, evaluating cells should be idempotent - that is, evaluating a cell multiple times should have the same effect as evaluating it once, ideally. Cells that are not idempotent can degrade the user experience.

Specifically, Go files tend to declare all imports in a single block. In a notebook, I'd put all imports in a single block, in their own cell. When I decide I need to import an additional package, I want to add that import to the existing cell and evaluate it. Without this MR, reevaluating that block usually causes an error.

interp/gta.go

@@ -209,9 +209,12 @@ func (interp *Interpreter) gta(root *node, rpath, importPath string) ([]*node, e
 					// imports in different source files of the same package. Therefore, we suffix
 					// the key with the basename of the source file.
 					name = filepath.Join(name, baseName)
-					if _, exists := sc.sym[name]; !exists {
+					if sym, exists := sc.sym[name]; !exists {
 						sc.sym[name] = &symbol{kind: pkgSym, typ: &itype{cat: binPkgT, path: ipath, scope: sc}}
 						break
+					} else if sym.kind == pkgSym && sym.typ.cat == srcPkgT && sym.typ.path == ipath {
+						// ignore re-import of identical package
+						break
 					}
 
 					// redeclaration error. Not caught by the parser.
@@ -233,9 +236,12 @@ func (interp *Interpreter) gta(root *node, rpath, importPath string) ([]*node, e
 						name = pkgName
 					}
 					name = filepath.Join(name, baseName)
-					if _, exists := sc.sym[name]; !exists {
+					if sym, exists := sc.sym[name]; !exists {
 						sc.sym[name] = &symbol{kind: pkgSym, typ: &itype{cat: srcPkgT, path: ipath, scope: sc}}
 						break
+					} else if sym.kind == pkgSym && sym.typ.cat == srcPkgT && sym.typ.path == ipath {
+						// ignore re-import of identical package
+						break
 					}
 
 					// redeclaration error