bug: Use manifest path as reported buildfile

[ethowitz]

Previously, we were reporting the workspace Cargo.toml as the buildfile
for a discover invocation. This led to incorrect behavior where RA
would "unindex" any crates that were included in the first discover
invocation but not in subsequent invocations (reporting the buildfile as
the workspace Cargo.toml implies that we are telling RA about all of the
crates in the workspace, but we are actually only telling it about a
subset). Consider the following dependency graph:

A: C
B: C
C:

If we're invoked with a file from crate A, we would tell RA about
crates A and C with the workspace manifest as the buildfile. If we
are subsequently invoked with a file from crate B, we'd still report
the buildfile as the workspace manifest, but now we'd only tell RA about
crates B and C. This causes RA to "unindex" A, since we're telling
RA that the workspace doesn't include it.

This commit rectifies the issue by reporting the current crate's
Cargo.toml as the buildfile in the discover output. However, this
changes the semantics of how and when check is invoked. In the above
example, we'd now report two separate "projects" after being invoked
with files from crates A and B. Because A and B do not depend on
each other, RA will invoke check separately for each of them. This is
rather annoying if they share many dependencies (e.g. if C has a giant
dependency tree), since those check invocations will be duplicating a
lot of work.

This behavior can be adjusted by setting RA's check.invocationStrategy
setting to "once", which tells it just to invoke check for the current
project instead of all the projects we've told it about. This commit
also updates the README to reflect this change.

Fixes #34