Remove the underscore prefix of __bp_imported

[FranklinYu]

This formally exposes it as part of the public API. RFC: @dimo414.

Fixes #122.

[rcaloras]

Thanks for the PR @FranklinYu! Looks like there's a couple other places that use the original name:

[rcaloras:~/git/bash-preexec] master ± git grep __bp_imported
...
test/include-test.bats:  __bp_imported="defined"
test/include-test.bats:  unset __bp_imported

Can you also the test case for this in to use the new variable name?

[FranklinYu]

All of the suggestion above looks good to me, but apparently the test is currently broken. For example, the “include test” use __bp_preexec_and_precmd_install to decide whether bash-preexec is loaded, but this function isn’t there regardless. Shall I start another PR to fix the test before moving forward with this one?

[FranklinYu]

I’m glad that the CI is working, and it caught a bug. I’ll fix it before marking this as “ready for review”.

[FranklinYu]

I finally addressed all the comments. The history seems a bit messy; maybe consider merge by squashing the commits.

[rcaloras]

LGTM 👍