Add support for rewriting source path location stored in PDB/MDB

Hi there,
This is a small enhancement to allow rewriting of sourcepath location for PDB/MDB before saving them. This is useful when we need to package sources with assemblies and PDBs in a new location on the disk (for example, I'm using it in a nuget package at deploy time).
The changes are small:
 - New delegate `string SourcePathRewriterDelegate(string)`
 - New property `WriteParameters.SourcePathRewriter`
 - Modifed `ISymbolWriterProvider` to pass this new parameters to their implems
 - Called from PdbWriter/MdbWriter