Allow to load patch from specific package

[johannes85]

This changes allows to load the patch from a specific packageb by defining its location in the following way: //package/name//patch/to/patch

For example:

"extra": {
        "patches": {
            "xp-framework/core": {
                "Safer Objects::stringOf": "//my/package//src/main/patches/xp-framework-core-src-main-php-util-objects-class-php.patch"
            }
        }
    },

I implemented this change because when you define a local patch file in a package like "patches/patch1.patch" the installation of this patch only works when executing composer in the package itself. When the package is a dependency of a project( with "enable-patching"), the patch file can't be found since the current root directory is another one.

[cweagans]

Probably not going to do this in 1.x, and 2.x doesn't have the idea of resolving patches from dependencies.