Refine launch process detection

[runesoerensen]

We currently add launch processes by default (since #48). This issue tracks efforts required to further improve the current implementation, which may be improved in a number of ways:

• Failing to detect a process type shouldn't cause a buildpack execution error. (addressed in Streamline error handling #91)
• We may want to allow buildpack users to opt-out of this functionality.
• No information is currently logged about this functionality. (addressed in Improve process registration logging #124)
• The constructed executable process path is based solely on a priori logic, factoring in only some of the factors that can affect the actual output path. For instance, a project's OutputPath and BaseOutputPath properties are not considered at this time. (addressed in Configure project publish dir #121, which overrides any OutputPath, BaseOutputPath, PublishDirconfiguration in project files).
• There's no validation that the executable process file actually exists at the expected path (after compiling the source repository) - or that it's executable.
• We may want to add validation/warnings when a process type conflicts (or can be confused) with that of another $PATH binary. For instance, in the (somewhat unlikely) scenario that the project/assembly name is bash, this would, among other things, result in docker run --rm -it --entrypoint bash my-image-name executing the compiled process rather than e.g. /usr/bin/bash, which may not be the expected/intended/desirable behavior.
• We may want to register a default "web" process type when it's safe to do so. In particular, when the source app directory only contain a single .NET web project file.