lockfile_generator module name cleanup

[kylewillmon]

Here are the current modules and the commands that they use to generate lockfiles

> grep -r 'Command::new' lockfile_generator/src
lockfile_generator/src/npm.rs:        let mut command = Command::new("npm");
lockfile_generator/src/poetry.rs:        let mut command = Command::new("poetry");
lockfile_generator/src/cargo.rs:        let mut command = Command::new("cargo");
lockfile_generator/src/bundler.rs:        let mut command = Command::new("bundle");
lockfile_generator/src/pip.rs:        let mut command = Command::new("pipenv");
lockfile_generator/src/python_requirements.rs:        let mut command = Command::new("pip-compile");
lockfile_generator/src/maven.rs:        let mut command = Command::new("mvn");
lockfile_generator/src/go.rs:        let mut command = Command::new("go");
lockfile_generator/src/gradle.rs:        let mut command = Command::new("gradle");
lockfile_generator/src/yarn.rs:        let mut command = Command::new("yarn");

pip.rs calling pipenv is particularly misleading. This issue will track improving these names.

Originally posted by @maxrake in #1075 (comment)

[kylewillmon]

If we name the files after the project that contains the tool, we would need to rename just these two files:

• pip.rs -> pipenv.rs
• python_requirements.rs -> pip_tools.rs