compiler/checker: investigate if importing the same name twice is handled correctly

[zapateo]

In the function checkImport we import the names without checking if they have already been imported. Not sure if this may lead to incosistencies, we should investigate on it.

NOTE: before proceeding, wait for the merge of the PR #856.

[zapateo]

Consider this scenario:

// main.go
package main

import (
	. "test/bar"
	. "test/foo"
)

func main() {
	Foo()
	Bar()
}

// bar/bar.go
package bar

func F() {}

func Bar() {}

// foo/foo.go
package foo

func F() {}

func Foo() {}

gc returns the error:

./main.go:5:2: F redeclared during import "test/foo"
        main.go:4:2: previous declaration during import "test/bar"

[zapateo]

We should consider and test the various combinations of native and non-native imported names. For example:

• first declaration imported is native, a second one (with the same name) is native too
• first declaration imported is native, a second one (with the same name) is not native

etc...

[zapateo]

This code does not return error:

"https://github.com/open2b/scriggo/issues/859": {
	sources: fstest.Files{
		"index.html": `
		{% import "foo.html" %}
		{% import "bar.html" %}

		{{ Foo() }}
		{{ Bar() }}
	`,
		"foo.html": `
		{% macro F() %}{% end macro %}
		{% macro Foo() %}{% end macro %}
	`,
		"bar.html": `
		{% macro F() %}{% end macro %}
		{% macro Bar() %}{% end macro %}
	`,
	},
},

Neither this:

"https://github.com/open2b/scriggo/issues/859": {
	sources: fstest.Files{
		"index.html": `
			{% import "foo.html" %}
			{% import "bar.html" %}`,
		"foo.html": `{% macro F() %}{% end macro %}`,
		"bar.html": `{% macro F() %}{% end macro %}`,
	},
},

They both should return a redeclaration error.