Refactor bashrc scripts sourcing

Creates two new helper functions __try_source and __source_env_files to simplify
__source_all_bashrcs.
Add profile-formats to profile_complex
package.bashrc: per profile, per-package bashrc mechanism

Profiles can define per-package bashrc files to be sourced before emerging.
Each line in package.bashrc must be an atom name then a list of space-delimited
bashrc files (stored in $profile/bashrc/).