add testcase for npm-style import (based on pr #275 by shibukawa-san)
implement the feature
add compile-error test
add failing test re import "*.jsx" against an npm-style file layout
take care of ```import "*.jsx"``` against npm-style file layout
add load-order test for npm-style layout
cache the parsed contents of package.json
Merge branch 'master' into kazuho/experiments/npm-support
add failing test
emit warning if npm module (with the same name) is loaded from different locations
Merge branch 'kazuho/experiments/npm-support' (note: experimental feature)