NAC3: inject fake module for experiments submitted by content

Description of Changes

NAC3 Artiq expects every function and class to be in a module which has a file. Kernels submitted by content do not have a file, but their source still needs to be sent to the compiler; so the fake module StringLoader is passed to the compiler. This basically follows the behavior of the legacy compiler as well.

Tested with a Kasli, by submitting kernels with artiq_client, with and without --content option.

Requires changes on the NAC3 side too.

Related Issue

Closes nac3#223

Type of Changes

	Type
✓	:bug: Bug fix

Steps (Choose relevant, delete irrelevant before submitting)

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m-labs / artiq