rosdep is designed to treat pip like an alternative system-level package
manager. Deviating from this approach is not easily achievable without a
significant rethinking of how pip packages are managed. In the meantime,
we can at least instruct users how to restore the prior functionality.

Rather than inject the environment variable / config on behalf of the
user, this change instructs them to make the necessary config changes
themself, keeping them informed of the change their making to the
system's new default.

Co-authored-by: Christophe Bedard <[email protected]>

Co-authored-by: Christophe Bedard <[email protected]>

…arning.

The fallback configuration was over-indented and would never be
checked.

Use monospace formatting when referring to the `rosdep` command /
executable name and simply 'rosdep' when referring to the project.

The preferred capitalization of rosdep is rosdep not Rosdep or ROSdep
(and certainly not ROSDep).

Co-authored-by: Christophe Bedard <[email protected]>

Co-authored-by: Christophe Bedard <[email protected]>

Co-authored-by: Christophe Bedard <[email protected]>

Co-authored-by: Christophe Bedard <[email protected]>

This is a fixup after two earlier changes inverted the conditional and
reformatted an internal check.

…ed environments.

Using the dict access method will raise a KeyError when the config file
is present but this section or value is missing.