Most tools have an executable they depend on. A command line tool or

script that performs the actual checks. As part of worker startup, each

tool's check_dependencies() is called in order to check that any and

all command line tools were satisfied.
This process was repetitive, and a bit flawed. A couple of tools needed

manual configuration in order to find the tools, but assumptions were made

about the presence of configuration keys in one of these cases, despite

never making that a part of the default configuration. Other tools had

no ability to customize the location of the command line script.
This change introduces a new exe_dependencies attribute, which

BaseTool.check_dependencies() will check. Each item will be checked

both against a new exe_paths environment variable and against $PATH,

allowing customization.
If found on $PATH, the path will be placed back in exe_paths,

allowing tools to directly look up the location. That both simplifies

their code and avoids an implicit lookup on $PATH every time the tool

is executed.

Unit tests pass.
Made changes to all the tools (not part of this change) and verified that

dependency resolution worked correctly, and that results were cached.