Refactor FX resolution into its own class (dotnet/core-setup#5582)

Refactor FX resolution into its own class (dotnet/core-setup#5582)

fx_muxer is a collection of muxer functionality, framework resolution and command line parsing.
This change moves the framework resolution out into its own fx_resolver.

In the fx_resolver, made some of the methods "this" methods so that they don't have to pass the framework maps in arguments.

No functional changes, just moving code around.

Commit migrated from https://github.com/dotnet/core-setup/commit/20f2752ccd2fdbb285a77d66306e5ad2cdb81f07