Move input root to sub directory of build directory

[Qinusty]

That said, one change we should consider making to Buildbarn at some point is that we do move the input root into some kind of subdirectory, reserving the top-level directory for other storage purposes. For example, stdout and stderr of build actions are currently stored as files inside the input root. So like this:

input_root
├── .stdout.txt
├── .stderr.txt
├── input_file.c
└── output_file.o

Though most build actions generally don't care about that, it's a bit sloppy. If a build action would try to tar its entire input root, it would also archive its own stdout/stderr logs. We should consider restructuring it to something like this:

build_directory
├── stdout.txt
├── stderr.txt
└── input_root
    ├── input_file.c
    └── output_file.o

Originally posted by @EdSchouten in #18 (comment)

[Qinusty]

When doing this, it will be important to ensure that the runner executes the action under the input_root to ensure any provided toolchains are used. Buildstream supplies a full sandbox environment as it's input_files and as such autoconf may be expected to exist under $PATH, which could be placed at /bin, /usr/bin etc.

[EdSchouten]

This has been fixed now. Builds take place in a root subdirectory. The stdout and stderr logs are no longer prefixed with a dot, making it easier to find them.