removed as it turned out to be useless

Error: "There are no .py[i] files in directory 'plugins'"

- run plugin tests after main test battery.
- integrate plugin lint checks in main lint checks

Since plugin tests were added in the main pytest job,
pytest test filters are forwarded to pytest calls for plugin tests.
It could happen that some pytest filter deselect all
the tests of a plugin, which makes pytest exiting with code 5.

This has been addressed by using the plugin 'pytest-custom-exit-code'.

…onstants

the private keys are generated temporarily before the tests' execution.

- run test battery with development version of the plugins
- fix freeze command test

for cosmos plugin, in 'test_sign_and_recover_message_public_key',
if we use a temporary key the test doesn't pass. Instead,
it passes with the old fixture.

the reason is that fetchai could not depend on cosmos because not published
yet.

also, add check in workflow to ensure cosmos_crypto.cosmos and fetchai_crypto._cosmos
have the same content.

add check in workflow to ensure consistency (module must be the same everywhere)

the reason is that we can't specify '--no-deps' flag when
installing the plugins with Pipenv; so
the `aea` is downloaded from PyPI instead of resolved locally.
This causes conflicting subdependency versions like 'ecdsa'
which is >=0.16 in plugins and '==0.15' in aea 0.10.0.

- add missing crypto plugin dependencies to several contract packages
- replace "XCrypto" with "XApi"

in workflow, coverage_checks:

- missing '|'
- plugin-py3.7-cov test environment should have been called, instead of py3.7-cov.