build: editable dependencies in Python breaks build command

Currently, `sam build` uses a requirements.txt file to install dependancies, if a dep is marked as editable (-e in the file), it causes the `build` command to break with:
`OSError: [Errno 30] Read-only file system: '/tmp/samcli/source/src'`

This could potentially causes issues in a number of situations, most notably, people who use the same requirements.txt file for development and for build.

Ideally, `sam build` would strip/sanitise the requirements.txt to remove any flags for the build command.

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SAM CLI Version: 0.8.0