Adjust how InstallToPath=false is extracted

[mallardduck]

The current behaviour of InstallToPath=false dumps the full extract into the consistent path. It does not however move ONLY the extracted and checksum verified file to the consistent path...it moves the full extract path.

This creates an issue with how I intended for hooks to be able to interact with things w/o needing config/tool state other than (essentially) name. This change rectifies that mistake.

[mallardduck]

Hmm. This solution does look solid for what we know RN. But I tried setting up Nix to test and the image doesn't build:

0.150 performing a single-user installation of Nix...
0.152 copying Nix to /nix/store...
0.152 ./install: line 170: cd: ./store: No such file or directory
0.152
0.153 ./install: line 192: ./.reginfo: No such file or directory
0.153 ./install: unable to register valid paths

This makes me suspect Nix tar needs the full path extracted not just the install script. Yes that is exactly what is what - the tar has many files.

So I will look into expanding things to allow InstallToPath=false tools to extract the directory rather than a single file.

Update: Ok - fixed that as well. InstallToPath=false has logic to allow setting up files correctly. Now to create a tool hook, users only need to know the tool name to find the files and interact with them via post hooks. If we do hit a need for users to know more info in the hooks then we can do #38

Ready for final review and merge.