Add support for loading mappings in reverse

--srg-in ^filename will load the mappings as if they were reversed.

This is a decoration on the filename instead of a separate command-
line option flag so it can apply to the particular mapping file,
when loading multiple mappings simultaneously.
Disable reading .srg 'PK:' mappings for now

Example from packaged.srg:

PK: . net/minecraft/src
PK: com com
PK: net net
PK: net/minecraft net/minecraft
PK: net/minecraft/client net/minecraft/client
PK: net/minecraft/server net/minecraft/server

SpecialSource package remaps currently are class prefix matches,
taking precedence over class remaps. So net->net would match all
net.* classes when using reversed MCP (for reobfuscation), causing
no classes to remap. However the current behavior is useful for MCPC+,
remapping entire package hierarchies. Probably both behaviors should be
supported at some point, but until they are, disabling .srg PK:.
Add reverse and output shade relocation flags

Removed the ^/@-style input specifications, in favor of
command-line options (which apply to all mappings loaded):

--in-shade-relocation: applies to srg-in input names
--out-shade-relocation: applies to srg-in output names
--reverse: reverses srg-in input/output names

Output shading relocation is useful if remapping through
--srg-in obf2cb.srg, and you want versioned names to match CB.