[WIP] support slow OS commands.

[cypherfox]

What does this do / why do we need it?

It allows setting two environment variables (DEP_CMD_TIMEOUT and DEP_EXP_CMD_TIMEOUT) to control the timeout for external commands to finish.
On slow machines or slow setups (remote FS, or in my case ubuntu on windows) the 10s default are not sufficient.

The change will make dep tolerate incomplete deletions of vendor directories in dependant packages. This is also an attempt to support slower operations.

Do you need help or clarification on anything?

Where should documentation go?

Which issue(s) does this PR fix?

It uses an idea presented in #1065 to circumvent the problem shown there. Does that count as a fix?

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[cypherfox]

I signed the CLA

[googlebot]

CLAs look good, thanks!

[sdboyer]

i think this is the same as #1028, no?

[cypherfox]

It is almost the same. While I have similar changes for setting the timeout via env-vars, I also included a toleration for the OS not deleting a directory quickly enough.

I will separate the two and add another PR

[sdboyer]

thanks!