Inform user to build dependencies/project to create sources generated by build scripts

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Environment

• IntelliJ Rust plugin version: 0.2.108.2762-193-nightly
• Rust toolchain version: 1.38.0 (625451e37 2019-09-23) x86_64-apple-darwin
• IDE name and version: IntelliJ IDEA 2019.3 EAP Ultimate Edition (IU-193.4386.10)
• Operating system: macOS 10.13.6

Problem description

#4542 finishes support of all common cases how include macro can be used. More precisely, it provides support of env("OUT_DIR") inside macro call. OUT_DIR variable is used by build scripts where they generated including code.

But if a user hasn't compiled a project yet, all code insight features don't work because the plugin relies on existence of generated code in file system. And it's not clear that something doesn't work because build scripts haven't generated code yet.

It would be great to solve it somehow. For example, the plugin can show a notification to a user that expected generated sources are not present on file system. Or we can have an option to automatically build project/libraries/build scripts (see rust-lang/cargo#7068 and rust-lang/cargo#7178) to have an actual version of generated source code.

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Currently, we use cargo check instead of cargo build-plan to get values of compile-time variables (as well as another compile-time information). And cargo check also runs all build scripts so all necessary files are generated and available for plugin analysis. So I think this issue can be closed