WIP: Hadrian: make the default rule CWD aware

tl;dr: With this patch, running 'build.sh' with no target has a different behaviour depending on where we run it from (the current working directory). If under a package directory, Hadrian will rebuild just that package. If under a test directory, Hadrian will run just the tests under that directory.

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This required putting a few pieces together:

• The build.* scripts are now more flexible, we can run them from somewhere else than $TOP. They figure out $TOP, get the directory from which they were called (1) and pass it all to the 'hadrian' executable.
• Hadrian takes a new argument, --target-dir, whose value ends up being (1) from above.
• If --target-dir is $TOP, we build a complete GHC distribution.
• If --target-dir is a (subdirectory of a) package's directory, we (re)build just that package (and what might be required to build it, if it's not there already).
• If --target-dir is a test directory (i.e it contains an all.T file), we run just the tests that live under the said test directory.

This patch introduces a new module, Rules.TargetDirs, which provides a mapping from path component lists to package/test dir targets using a trie, generated from the pkgPath of all known packages as well as all the directories anywhere under $TOP that contain all.T files.

Note: this also includes the commit from !945 (merged) as it is required to implement the --target-dir is a test directory case. I will remove the WIP status and rebase/clean up this branch once !945 (merged) lands.