Import earlgrey target

[cfrantz]

This PR imports the pigweed target definitions for the OpenTitan Earlgrey chip.

1. Make the repo in to a bazel module and reference the upstream pigweed repo and language/compiler bazel rules repos.
2. Remove the /target/-type ignores from .gitignore. We'll have to consider how we want to handle this as we migrate this repo from being cargo-based to bazel-based. I'd be OK with renaming //target/ealrgrey to something else; this is currently the pigweed convention.
3. Move the original root-level //tooling and //signing subdirs into //target/earlgrey. Both of these subdirectories have earlgrey-specific definitions and probably shouldn't live at the root of the repo.

This is a draft PR initially meant to create some discussion. Namely:

• How do we want to organize the repository?
• How do we migrate from cargo/xtask to bazel for existing flows?
• What additional automation do we want to put in place in the repo?
• What form of license header do we want to apply to files? (currently the imported files say "copyright pigweed authors")

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