[mkbundle] support satellite assemblies (#3448)

[mkbundle] support satellite assemblies (#3448)

Satellite assemblies are special in that they all share the same file
name, but they live in separate directories named after the locale the
satellite assemblies pack. mkbundle currently doesn't support "clashing"
assembly filenames and will not be able to store the satellite
assemblies in the bundle for that reason.

This patch adds support for storing satellite assemblies with their
culture path prefix (e.g. en-US/MyAssembly.resources.dll) in the bundle
and also special code in the assembly loader to support such filenames
in the code which loads assemblies from the bundle.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=27061