We should allow callers of MinidumpProcessor to supply their own version of SourceLineResolver, providing the current implementation as a base to build on. This would be useful in any situation where loading a full symbol file into the calling process is not desired.

Still TBD: if we do this, is the SymbolSupplier interface still useful? It would be possible for a custom implementation of SourceLineResolver to load symbol files any way that it wants to, without having to return a local file path as a string.

This patch factors out an abstract interface to SourceLineResolver, and makes the header for the concrete implementation public. I didn't make any changes to the SymbolSupplier functionality.

patch version 2