Separate package and module names from dslx source file names

Currently during ir conversion, the dslx source file path may be used to create package and module names.

https://github.com/google/xls/blob/main/xls/dslx/ir_convert/ir_converter.cc#L3717
https://github.com/google/xls/blob/main/xls/dslx/ir_convert/ir_converter.cc#L3730

This means that dslx file names .x must be prefixed (before the dot) with valid dslx identifier names.

This issue is to relax this constraint so that dslx files can be named to be any valid filesystem name.