Replace slashes with underscores in default crate names.

[cfredric]

This is another attempt at #1334, which sought to make "/" characters legal in Rust target names.

This PR differs from #1334 in how it handles build artifacts that are based on crate_info.output. There are a few places that currently assume that calling ctx.actions.declare_file(...) will always declare a file in the same directory as crate_info.output, i.e. as ctx.actions.declare_file(ctx.label.name + ...). In a world where label names may include slashes (which is the case today!), this is not a safe assumption, since a slash in the target name causes bazel to create a subdirectory for the output.

Specifically, this PR fixes incorrect locations for the following items (when the target name includes a slash):

• PDB files
• dSYM files
• .lib files

This leaves a few things that look like they make the same assumption, but I'm not sure how they're used or if they should be updated:

• "_ambiguous_libs/" + crate_info.output.basename + "/" + symlink_name for ambiguous libraries
• The linkstamp output path ("_objs/" + crate_info.output.basename + "/" + ...)

[scentini]

Thanks!